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Documentation: convert path-lookup from markdown to resturctured text 

This allows the document to be integrated with the main documentation
tree.
Changes include:
- rename from .md to .rst
- use `` for code, not single `
- use correct sub-section marking
- fix indented blocks, both code and non-code
- fix external-link markup
Signed-off-by: default avatarNeilBrown <neilb@suse.com>
[jc: changed the toctree organization a bit]
Signed-off-by: default avatarJonathan Corbet <corbet@lwn.net>
parent 036c20c0
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Documentation/filesystems/index.rst
View file @ 7bbfd9ad
...@@ -359,3 +359,14 @@ encryption of files and directories. ...@@ -359,3 +359,14 @@ encryption of files and directories.
:maxdepth: 2 :maxdepth: 2
fscrypt fscrypt
Pathname lookup
===============
Pathname lookup in Linux is a complex beast; the document linked below
provides a comprehensive summary for those looking for the details.
.. toctree::
:maxdepth: 2
path-lookup.rst
Documentation/filesystems/path-lookup.md → Documentation/filesystems/path-lookup.rst
View file @ 7bbfd9ad
<head> ========================
<style> p { max-width:50em} ol, ul {max-width: 40em}</style> Pathname lookup in Linux
</head> ========================
Pathname lookup in Linux.
=========================
This write-up is based on three articles published at lwn.net: This write-up is based on three articles published at lwn.net:
...@@ -17,8 +14,8 @@ including: ...@@ -17,8 +14,8 @@ including:
- per-directory parallel name lookup. - per-directory parallel name lookup.
Introduction Introduction to pathname lookup
------------ ===============================
The most obvious aspect of pathname lookup, which very little The most obvious aspect of pathname lookup, which very little
exploration is needed to discover, is that it is complex. There are exploration is needed to discover, is that it is complex. There are
...@@ -36,58 +33,58 @@ distinctions we need to clarify first. ...@@ -36,58 +33,58 @@ distinctions we need to clarify first.
There are two sorts of ... There are two sorts of ...
-------------------------- --------------------------
[`openat()`]: http://man7.org/linux/man-pages/man2/openat.2.html .. _openat: http://man7.org/linux/man-pages/man2/openat.2.html
Pathnames (sometimes "file names"), used to identify objects in the Pathnames (sometimes "file names"), used to identify objects in the
filesystem, will be familiar to most readers. They contain two sorts filesystem, will be familiar to most readers. They contain two sorts
of elements: "slashes" that are sequences of one or more "`/`" of elements: "slashes" that are sequences of one or more "``/``"
characters, and "components" that are sequences of one or more characters, and "components" that are sequences of one or more
non-"`/`" characters. These form two kinds of paths. Those that non-"``/``" characters. These form two kinds of paths. Those that
start with slashes are "absolute" and start from the filesystem root. start with slashes are "absolute" and start from the filesystem root.
The others are "relative" and start from the current directory, or The others are "relative" and start from the current directory, or
from some other location specified by a file descriptor given to a from some other location specified by a file descriptor given to a
"xxx`at`" system call such as "[`openat()`]". "``XXXat``" system call such as `openat() <openat_>`_.
[`execveat()`]: http://man7.org/linux/man-pages/man2/execveat.2.html .. _execveat: http://man7.org/linux/man-pages/man2/execveat.2.html
It is tempting to describe the second kind as starting with a It is tempting to describe the second kind as starting with a
component, but that isn't always accurate: a pathname can lack both component, but that isn't always accurate: a pathname can lack both
slashes and components, it can be empty, in other words. This is slashes and components, it can be empty, in other words. This is
generally forbidden in POSIX, but some of those "xxx`at`" system calls generally forbidden in POSIX, but some of those "xxx``at``" system calls
in Linux permit it when the `AT_EMPTY_PATH` flag is given. For in Linux permit it when the ``AT_EMPTY_PATH`` flag is given. For
example, if you have an open file descriptor on an executable file you example, if you have an open file descriptor on an executable file you
can execute it by calling [`execveat()`] passing the file descriptor, can execute it by calling `execveat() <execveat_>`_ passing
an empty path, and the `AT_EMPTY_PATH` flag. the file descriptor, an empty path, and the ``AT_EMPTY_PATH`` flag.
These paths can be divided into two sections: the final component and These paths can be divided into two sections: the final component and
everything else. The "everything else" is the easy bit. In all cases everything else. The "everything else" is the easy bit. In all cases
it must identify a directory that already exists, otherwise an error it must identify a directory that already exists, otherwise an error
such as `ENOENT` or `ENOTDIR` will be reported. such as ``ENOENT`` or ``ENOTDIR`` will be reported.
The final component is not so simple. Not only do different system The final component is not so simple. Not only do different system
calls interpret it quite differently (e.g. some create it, some do calls interpret it quite differently (e.g. some create it, some do
not), but it might not even exist: neither the empty pathname nor the not), but it might not even exist: neither the empty pathname nor the
pathname that is just slashes have a final component. If it does pathname that is just slashes have a final component. If it does
exist, it could be "`.`" or "`..`" which are handled quite differently exist, it could be "``.``" or "``..``" which are handled quite differently
from other components. from other components.
[POSIX]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_12 .. _POSIX: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_12
If a pathname ends with a slash, such as "`/tmp/foo/`" it might be If a pathname ends with a slash, such as "``/tmp/foo/``" it might be
tempting to consider that to have an empty final component. In many tempting to consider that to have an empty final component. In many
ways that would lead to correct results, but not always. In ways that would lead to correct results, but not always. In
particular, `mkdir()` and `rmdir()` each create or remove a directory named particular, ``mkdir()`` and ``rmdir()`` each create or remove a directory named
by the final component, and they are required to work with pathnames by the final component, and they are required to work with pathnames
ending in "`/`". According to [POSIX] ending in "``/``". According to POSIX_
> A pathname that contains at least one non- &lt;slash> character and A pathname that contains at least one non- &lt;slash> character and
> that ends with one or more trailing &lt;slash> characters shall not that ends with one or more trailing &lt;slash> characters shall not
> be resolved successfully unless the last pathname component before be resolved successfully unless the last pathname component before
> the trailing <slash> characters names an existing directory or a the trailing <slash> characters names an existing directory or a
> directory entry that is to be created for a directory immediately directory entry that is to be created for a directory immediately
> after the pathname is resolved. after the pathname is resolved.
The Linux pathname walking code (mostly in `fs/namei.c`) deals with The Linux pathname walking code (mostly in ``fs/namei.c``) deals with
all of these issues: breaking the path into components, handling the all of these issues: breaking the path into components, handling the
"everything else" quite separately from the final component, and "everything else" quite separately from the final component, and
checking that the trailing slash is not used where it isn't checking that the trailing slash is not used where it isn't
...@@ -104,15 +101,15 @@ of the possible races are seen most clearly in the context of the ...@@ -104,15 +101,15 @@ of the possible races are seen most clearly in the context of the
"dcache" and an understanding of that is central to understanding "dcache" and an understanding of that is central to understanding
pathname lookup. pathname lookup.
More than just a cache. More than just a cache
----------------------- ----------------------
The "dcache" caches information about names in each filesystem to The "dcache" caches information about names in each filesystem to
make them quickly available for lookup. Each entry (known as a make them quickly available for lookup. Each entry (known as a
"dentry") contains three significant fields: a component name, a "dentry") contains three significant fields: a component name, a
pointer to a parent dentry, and a pointer to the "inode" which pointer to a parent dentry, and a pointer to the "inode" which
contains further information about the object in that parent with contains further information about the object in that parent with
the given name. The inode pointer can be `NULL` indicating that the the given name. The inode pointer can be ``NULL`` indicating that the
name doesn't exist in the parent. While there can be linkage in the name doesn't exist in the parent. While there can be linkage in the
dentry of a directory to the dentries of the children, that linkage is dentry of a directory to the dentries of the children, that linkage is
not used for pathname lookup, and so will not be considered here. not used for pathname lookup, and so will not be considered here.
...@@ -139,7 +136,7 @@ whether remote filesystems like NFS and 9P, or cluster filesystems ...@@ -139,7 +136,7 @@ whether remote filesystems like NFS and 9P, or cluster filesystems
like ocfs2 or cephfs. These filesystems allow the VFS to revalidate like ocfs2 or cephfs. These filesystems allow the VFS to revalidate
cached information, and must provide their own protection against cached information, and must provide their own protection against
awkward races. The VFS can detect these filesystems by the awkward races. The VFS can detect these filesystems by the
`DCACHE_OP_REVALIDATE` flag being set in the dentry. ``DCACHE_OP_REVALIDATE`` flag being set in the dentry.
REF-walk: simple concurrency management with refcounts and spinlocks REF-walk: simple concurrency management with refcounts and spinlocks
-------------------------------------------------------------------- --------------------------------------------------------------------
...@@ -148,22 +145,23 @@ With all of those divisions carefully classified, we can now start ...@@ -148,22 +145,23 @@ With all of those divisions carefully classified, we can now start
looking at the actual process of walking along a path. In particular looking at the actual process of walking along a path. In particular
we will start with the handling of the "everything else" part of a we will start with the handling of the "everything else" part of a
pathname, and focus on the "REF-walk" approach to concurrency pathname, and focus on the "REF-walk" approach to concurrency
management. This code is found in the `link_path_walk()` function, if management. This code is found in the ``link_path_walk()`` function, if
you ignore all the places that only run when "`LOOKUP_RCU`" you ignore all the places that only run when "``LOOKUP_RCU``"
(indicating the use of RCU-walk) is set. (indicating the use of RCU-walk) is set.
[Meet the Lockers]: https://lwn.net/Articles/453685/ .. _Meet the Lockers: https://lwn.net/Articles/453685/
REF-walk is fairly heavy-handed with locks and reference counts. Not REF-walk is fairly heavy-handed with locks and reference counts. Not
as heavy-handed as in the old "big kernel lock" days, but certainly not as heavy-handed as in the old "big kernel lock" days, but certainly not
afraid of taking a lock when one is needed. It uses a variety of afraid of taking a lock when one is needed. It uses a variety of
different concurrency controls. A background understanding of the different concurrency controls. A background understanding of the
various primitives is assumed, or can be gleaned from elsewhere such various primitives is assumed, or can be gleaned from elsewhere such
as in [Meet the Lockers]. as in `Meet the Lockers`_.
The locking mechanisms used by REF-walk include: The locking mechanisms used by REF-walk include:
### dentry->d_lockref ### dentry->d_lockref
~~~~~~~~~~~~~~~~~
This uses the lockref primitive to provide both a spinlock and a This uses the lockref primitive to provide both a spinlock and a
reference count. The special-sauce of this primitive is that the reference count. The special-sauce of this primitive is that the
...@@ -172,49 +170,51 @@ with a single atomic memory operation. ...@@ -172,49 +170,51 @@ with a single atomic memory operation.
Holding a reference on a dentry ensures that the dentry won't suddenly Holding a reference on a dentry ensures that the dentry won't suddenly
be freed and used for something else, so the values in various fields be freed and used for something else, so the values in various fields
will behave as expected. It also protects the `->d_inode` reference will behave as expected. It also protects the ``->d_inode`` reference
to the inode to some extent. to the inode to some extent.
The association between a dentry and its inode is fairly permanent. The association between a dentry and its inode is fairly permanent.
For example, when a file is renamed, the dentry and inode move For example, when a file is renamed, the dentry and inode move
together to the new location. When a file is created the dentry will together to the new location. When a file is created the dentry will
initially be negative (i.e. `d_inode` is `NULL`), and will be assigned initially be negative (i.e. ``d_inode`` is ``NULL``), and will be assigned
to the new inode as part of the act of creation. to the new inode as part of the act of creation.
When a file is deleted, this can be reflected in the cache either by When a file is deleted, this can be reflected in the cache either by
setting `d_inode` to `NULL`, or by removing it from the hash table setting ``d_inode`` to ``NULL``, or by removing it from the hash table
(described shortly) used to look up the name in the parent directory. (described shortly) used to look up the name in the parent directory.
If the dentry is still in use the second option is used as it is If the dentry is still in use the second option is used as it is
perfectly legal to keep using an open file after it has been deleted perfectly legal to keep using an open file after it has been deleted
and having the dentry around helps. If the dentry is not otherwise in and having the dentry around helps. If the dentry is not otherwise in
use (i.e. if the refcount in `d_lockref` is one), only then will use (i.e. if the refcount in ``d_lockref`` is one), only then will
`d_inode` be set to `NULL`. Doing it this way is more efficient for a ``d_inode`` be set to ``NULL``. Doing it this way is more efficient for a
very common case. very common case.
So as long as a counted reference is held to a dentry, a non-`NULL` `->d_inode` So as long as a counted reference is held to a dentry, a non-``NULL`` ``->d_inode``
value will never be changed. value will never be changed.
### dentry->d_lock ### dentry->d_lock
~~~~~~~~~~~~~~
`d_lock` is a synonym for the spinlock that is part of `d_lockref` above. ``d_lock`` is a synonym for the spinlock that is part of ``d_lockref`` above.
For our purposes, holding this lock protects against the dentry being For our purposes, holding this lock protects against the dentry being
renamed or unlinked. In particular, its parent (`d_parent`), and its renamed or unlinked. In particular, its parent (``d_parent``), and its
name (`d_name`) cannot be changed, and it cannot be removed from the name (``d_name``) cannot be changed, and it cannot be removed from the
dentry hash table. dentry hash table.
When looking for a name in a directory, REF-walk takes `d_lock` on When looking for a name in a directory, REF-walk takes ``d_lock`` on
each candidate dentry that it finds in the hash table and then checks each candidate dentry that it finds in the hash table and then checks
that the parent and name are correct. So it doesn't lock the parent that the parent and name are correct. So it doesn't lock the parent
while searching in the cache; it only locks children. while searching in the cache; it only locks children.
When looking for the parent for a given name (to handle "`..`"), When looking for the parent for a given name (to handle "``..``"),
REF-walk can take `d_lock` to get a stable reference to `d_parent`, REF-walk can take ``d_lock`` to get a stable reference to ``d_parent``,
but it first tries a more lightweight approach. As seen in but it first tries a more lightweight approach. As seen in
`dget_parent()`, if a reference can be claimed on the parent, and if ``dget_parent()``, if a reference can be claimed on the parent, and if
subsequently `d_parent` can be seen to have not changed, then there is subsequently ``d_parent`` can be seen to have not changed, then there is
no need to actually take the lock on the child. no need to actually take the lock on the child.
### rename_lock ### rename_lock
~~~~~~~~~~~
Looking up a given name in a given directory involves computing a hash Looking up a given name in a given directory involves computing a hash
from the two values (the name and the dentry of the directory), from the two values (the name and the dentry of the directory),
...@@ -228,114 +228,117 @@ happened to be looking at a dentry that was moved in this way, ...@@ -228,114 +228,117 @@ happened to be looking at a dentry that was moved in this way,
it might end up continuing the search down the wrong chain, it might end up continuing the search down the wrong chain,
and so miss out on part of the correct chain. and so miss out on part of the correct chain.
The name-lookup process (`d_lookup()`) does _not_ try to prevent this The name-lookup process (``d_lookup()``) does _not_ try to prevent this
from happening, but only to detect when it happens. from happening, but only to detect when it happens.
`rename_lock` is a seqlock that is updated whenever any dentry is ``rename_lock`` is a seqlock that is updated whenever any dentry is
renamed. If `d_lookup` finds that a rename happened while it renamed. If ``d_lookup`` finds that a rename happened while it
unsuccessfully scanned a chain in the hash table, it simply tries unsuccessfully scanned a chain in the hash table, it simply tries
again. again.
### inode->i_rwsem ### inode->i_rwsem
~~~~~~~~~~~~~~
`i_rwsem` is a read/write semaphore that serializes all changes to a particular ``i_rwsem`` is a read/write semaphore that serializes all changes to a particular
directory. This ensures that, for example, an `unlink()` and a `rename()` directory. This ensures that, for example, an ``unlink()`` and a ``rename()``
cannot both happen at the same time. It also keeps the directory cannot both happen at the same time. It also keeps the directory
stable while the filesystem is asked to look up a name that is not stable while the filesystem is asked to look up a name that is not
currently in the dcache or, optionally, when the list of entries in a currently in the dcache or, optionally, when the list of entries in a
directory is being retrieved with `readdir()`. directory is being retrieved with ``readdir()``.
This has a complementary role to that of `d_lock`: `i_rwsem` on a This has a complementary role to that of ``d_lock``: ``i_rwsem`` on a
directory protects all of the names in that directory, while `d_lock` directory protects all of the names in that directory, while ``d_lock``
on a name protects just one name in a directory. Most changes to the on a name protects just one name in a directory. Most changes to the
dcache hold `i_rwsem` on the relevant directory inode and briefly take dcache hold ``i_rwsem`` on the relevant directory inode and briefly take
`d_lock` on one or more the dentries while the change happens. One ``d_lock`` on one or more the dentries while the change happens. One
exception is when idle dentries are removed from the dcache due to exception is when idle dentries are removed from the dcache due to
memory pressure. This uses `d_lock`, but `i_rwsem` plays no role. memory pressure. This uses ``d_lock``, but ``i_rwsem`` plays no role.
The semaphore affects pathname lookup in two distinct ways. Firstly it The semaphore affects pathname lookup in two distinct ways. Firstly it
prevents changes during lookup of a name in a directory. `walk_component()` uses prevents changes during lookup of a name in a directory. ``walk_component()`` uses
`lookup_fast()` first which, in turn, checks to see if the name is in the cache, ``lookup_fast()`` first which, in turn, checks to see if the name is in the cache,
using only `d_lock` locking. If the name isn't found, then `walk_component()` using only ``d_lock`` locking. If the name isn't found, then ``walk_component()``
falls back to `lookup_slow()` which takes a shared lock on `i_rwsem`, checks again that falls back to ``lookup_slow()`` which takes a shared lock on ``i_rwsem``, checks again that
the name isn't in the cache, and then calls in to the filesystem to get a the name isn't in the cache, and then calls in to the filesystem to get a
definitive answer. A new dentry will be added to the cache regardless of definitive answer. A new dentry will be added to the cache regardless of
the result. the result.
Secondly, when pathname lookup reaches the final component, it will Secondly, when pathname lookup reaches the final component, it will
sometimes need to take an exclusive lock on `i_rwsem` before performing the last lookup so sometimes need to take an exclusive lock on ``i_rwsem`` before performing the last lookup so
that the required exclusion can be achieved. How path lookup chooses that the required exclusion can be achieved. How path lookup chooses
to take, or not take, `i_rwsem` is one of the to take, or not take, ``i_rwsem`` is one of the
issues addressed in a subsequent section. issues addressed in a subsequent section.
If two threads attempt to look up the same name at the same time - a If two threads attempt to look up the same name at the same time - a
name that is not yet in the dcache - the shared lock on `i_rwsem` will name that is not yet in the dcache - the shared lock on ``i_rwsem`` will
not prevent them both adding new dentries with the same name. As this not prevent them both adding new dentries with the same name. As this
would result in confusion an extra level of interlocking is used, would result in confusion an extra level of interlocking is used,
based around a secondary hash table (`in_lookup_hashtable`) and a based around a secondary hash table (``in_lookup_hashtable``) and a
per-dentry flag bit (`DCACHE_PAR_LOOKUP`). per-dentry flag bit (``DCACHE_PAR_LOOKUP``).
To add a new dentry to the cache while only holding a shared lock on To add a new dentry to the cache while only holding a shared lock on
`i_rwsem`, a thread must call `d_alloc_parallel()`. This allocates a ``i_rwsem``, a thread must call ``d_alloc_parallel()``. This allocates a
dentry, stores the required name and parent in it, checks if there dentry, stores the required name and parent in it, checks if there
is already a matching dentry in the primary or secondary hash is already a matching dentry in the primary or secondary hash
tables, and if not, stores the newly allocated dentry in the secondary tables, and if not, stores the newly allocated dentry in the secondary
hash table, with `DCACHE_PAR_LOOKUP` set. hash table, with ``DCACHE_PAR_LOOKUP`` set.
If a matching dentry was found in the primary hash table then that is If a matching dentry was found in the primary hash table then that is
returned and the caller can know that it lost a race with some other returned and the caller can know that it lost a race with some other
thread adding the entry. If no matching dentry is found in either thread adding the entry. If no matching dentry is found in either
cache, the newly allocated dentry is returned and the caller can cache, the newly allocated dentry is returned and the caller can
detect this from the presence of `DCACHE_PAR_LOOKUP`. In this case it detect this from the presence of ``DCACHE_PAR_LOOKUP``. In this case it
knows that it has won any race and now is responsible for asking the knows that it has won any race and now is responsible for asking the
filesystem to perform the lookup and find the matching inode. When filesystem to perform the lookup and find the matching inode. When
the lookup is complete, it must call `d_lookup_done()` which clears the lookup is complete, it must call ``d_lookup_done()`` which clears
the flag and does some other house keeping, including removing the the flag and does some other house keeping, including removing the
dentry from the secondary hash table - it will normally have been dentry from the secondary hash table - it will normally have been
added to the primary hash table already. Note that a `struct added to the primary hash table already. Note that a ``struct
waitqueue_head` is passed to `d_alloc_parallel()`, and waitqueue_head`` is passed to ``d_alloc_parallel()``, and
`d_lookup_done()` must be called while this `waitqueue_head` is still ``d_lookup_done()`` must be called while this ``waitqueue_head`` is still
in scope. in scope.
If a matching dentry is found in the secondary hash table, If a matching dentry is found in the secondary hash table,
`d_alloc_parallel()` has a little more work to do. It first waits for ``d_alloc_parallel()`` has a little more work to do. It first waits for
`DCACHE_PAR_LOOKUP` to be cleared, using a wait_queue that was passed ``DCACHE_PAR_LOOKUP`` to be cleared, using a wait_queue that was passed
to the instance of `d_alloc_parallel()` that won the race and that to the instance of ``d_alloc_parallel()`` that won the race and that
will be woken by the call to `d_lookup_done()`. It then checks to see will be woken by the call to ``d_lookup_done()``. It then checks to see
if the dentry has now been added to the primary hash table. If it if the dentry has now been added to the primary hash table. If it
has, the dentry is returned and the caller just sees that it lost any has, the dentry is returned and the caller just sees that it lost any
race. If it hasn't been added to the primary hash table, the most race. If it hasn't been added to the primary hash table, the most
likely explanation is that some other dentry was added instead using likely explanation is that some other dentry was added instead using
`d_splice_alias()`. In any case, `d_alloc_parallel()` repeats all the ``d_splice_alias()``. In any case, ``d_alloc_parallel()`` repeats all the
look ups from the start and will normally return something from the look ups from the start and will normally return something from the
primary hash table. primary hash table.
### mnt->mnt_count ### mnt->mnt_count
~~~~~~~~~~~~~~
`mnt_count` is a per-CPU reference counter on "`mount`" structures. ``mnt_count`` is a per-CPU reference counter on "``mount``" structures.
Per-CPU here means that incrementing the count is cheap as it only Per-CPU here means that incrementing the count is cheap as it only
uses CPU-local memory, but checking if the count is zero is expensive as uses CPU-local memory, but checking if the count is zero is expensive as
it needs to check with every CPU. Taking a `mnt_count` reference it needs to check with every CPU. Taking a ``mnt_count`` reference
prevents the mount structure from disappearing as the result of regular prevents the mount structure from disappearing as the result of regular
unmount operations, but does not prevent a "lazy" unmount. So holding unmount operations, but does not prevent a "lazy" unmount. So holding
`mnt_count` doesn't ensure that the mount remains in the namespace and, ``mnt_count`` doesn't ensure that the mount remains in the namespace and,
in particular, doesn't stabilize the link to the mounted-on dentry. It in particular, doesn't stabilize the link to the mounted-on dentry. It
does, however, ensure that the `mount` data structure remains coherent, does, however, ensure that the ``mount`` data structure remains coherent,
and it provides a reference to the root dentry of the mounted and it provides a reference to the root dentry of the mounted
filesystem. So a reference through `->mnt_count` provides a stable filesystem. So a reference through ``->mnt_count`` provides a stable
reference to the mounted dentry, but not the mounted-on dentry. reference to the mounted dentry, but not the mounted-on dentry.
### mount_lock ### mount_lock
~~~~~~~~~~
`mount_lock` is a global seqlock, a bit like `rename_lock`. It can be used to ``mount_lock`` is a global seqlock, a bit like ``rename_lock``. It can be used to
check if any change has been made to any mount points. check if any change has been made to any mount points.
While walking down the tree (away from the root) this lock is used when While walking down the tree (away from the root) this lock is used when
crossing a mount point to check that the crossing was safe. That is, crossing a mount point to check that the crossing was safe. That is,
the value in the seqlock is read, then the code finds the mount that the value in the seqlock is read, then the code finds the mount that
is mounted on the current directory, if there is one, and increments is mounted on the current directory, if there is one, and increments
the `mnt_count`. Finally the value in `mount_lock` is checked against the ``mnt_count``. Finally the value in ``mount_lock`` is checked against
the old value. If there is no change, then the crossing was safe. If there the old value. If there is no change, then the crossing was safe. If there
was a change, the `mnt_count` is decremented and the whole process is was a change, the ``mnt_count`` is decremented and the whole process is
retried. retried.
When walking up the tree (towards the root) by following a ".." link, When walking up the tree (towards the root) by following a ".." link,
...@@ -345,7 +348,8 @@ any changes to any mount points while stepping up. This locking is ...@@ -345,7 +348,8 @@ any changes to any mount points while stepping up. This locking is
needed to stabilize the link to the mounted-on dentry, which the needed to stabilize the link to the mounted-on dentry, which the
refcount on the mount itself doesn't ensure. refcount on the mount itself doesn't ensure.
### RCU ### RCU
~~~
Finally the global (but extremely lightweight) RCU read lock is held Finally the global (but extremely lightweight) RCU read lock is held
from time to time to ensure certain data structures don't get freed from time to time to ensure certain data structures don't get freed
...@@ -354,137 +358,141 @@ unexpectedly. ...@@ -354,137 +358,141 @@ unexpectedly.
In particular it is held while scanning chains in the dcache hash In particular it is held while scanning chains in the dcache hash
table, and the mount point hash table. table, and the mount point hash table.
Bringing it together with `struct nameidata` Bringing it together with ``struct nameidata``
-------------------------------------------- --------------------------------------------
[First edition Unix]: http://minnie.tuhs.org/cgi-bin/utree.pl?file=V1/u2.s .. _First edition Unix: http://minnie.tuhs.org/cgi-bin/utree.pl?file=V1/u2.s
Throughout the process of walking a path, the current status is stored Throughout the process of walking a path, the current status is stored
in a `struct nameidata`, "namei" being the traditional name - dating in a ``struct nameidata``, "namei" being the traditional name - dating
all the way back to [First Edition Unix] - of the function that all the way back to `First Edition Unix`_ - of the function that
converts a "name" to an "inode". `struct nameidata` contains (among converts a "name" to an "inode". ``struct nameidata`` contains (among
other fields): other fields):
### `struct path path` ### ``struct path path``
~~~~~~~~~~~~~~~~~~
A `path` contains a `struct vfsmount` (which is A ``path`` contains a ``struct vfsmount`` (which is
embedded in a `struct mount`) and a `struct dentry`. Together these embedded in a ``struct mount``) and a ``struct dentry``. Together these
record the current status of the walk. They start out referring to the record the current status of the walk. They start out referring to the
starting point (the current working directory, the root directory, or some other starting point (the current working directory, the root directory, or some other
directory identified by a file descriptor), and are updated on each directory identified by a file descriptor), and are updated on each
step. A reference through `d_lockref` and `mnt_count` is always step. A reference through ``d_lockref`` and ``mnt_count`` is always
held. held.
### `struct qstr last` ### ``struct qstr last``
~~~~~~~~~~~~~~~~~~
This is a string together with a length (i.e. _not_ `nul` terminated) This is a string together with a length (i.e. _not_ ``nul`` terminated)
that is the "next" component in the pathname. that is the "next" component in the pathname.
### `int last_type` ### ``int last_type``
~~~~~~~~~~~~~~~
This is one of `LAST_NORM`, `LAST_ROOT`, `LAST_DOT`, `LAST_DOTDOT`, or This is one of ``LAST_NORM``, ``LAST_ROOT``, ``LAST_DOT``, ``LAST_DOTDOT``, or
`LAST_BIND`. The `last` field is only valid if the type is ``LAST_BIND``. The ``last`` field is only valid if the type is
`LAST_NORM`. `LAST_BIND` is used when following a symlink and no ``LAST_NORM``. ``LAST_BIND`` is used when following a symlink and no
components of the symlink have been processed yet. Others should be components of the symlink have been processed yet. Others should be
fairly self-explanatory. fairly self-explanatory.
### `struct path root` ### ``struct path root``
~~~~~~~~~~~~~~~~~~
This is used to hold a reference to the effective root of the This is used to hold a reference to the effective root of the
filesystem. Often that reference won't be needed, so this field is filesystem. Often that reference won't be needed, so this field is
only assigned the first time it is used, or when a non-standard root only assigned the first time it is used, or when a non-standard root
is requested. Keeping a reference in the `nameidata` ensures that is requested. Keeping a reference in the ``nameidata`` ensures that
only one root is in effect for the entire path walk, even if it races only one root is in effect for the entire path walk, even if it races
with a `chroot()` system call. with a ``chroot()`` system call.
The root is needed when either of two conditions holds: (1) either the The root is needed when either of two conditions holds: (1) either the
pathname or a symbolic link starts with a "'/'", or (2) a "`..`" pathname or a symbolic link starts with a "'/'", or (2) a "``..``"
component is being handled, since "`..`" from the root must always stay component is being handled, since "``..``" from the root must always stay
at the root. The value used is usually the current root directory of at the root. The value used is usually the current root directory of
the calling process. An alternate root can be provided as when the calling process. An alternate root can be provided as when
`sysctl()` calls `file_open_root()`, and when NFSv4 or Btrfs call ``sysctl()`` calls ``file_open_root()``, and when NFSv4 or Btrfs call
`mount_subtree()`. In each case a pathname is being looked up in a very ``mount_subtree()``. In each case a pathname is being looked up in a very
specific part of the filesystem, and the lookup must not be allowed to specific part of the filesystem, and the lookup must not be allowed to
escape that subtree. It works a bit like a local `chroot()`. escape that subtree. It works a bit like a local ``chroot()``.
Ignoring the handling of symbolic links, we can now describe the Ignoring the handling of symbolic links, we can now describe the
"`link_path_walk()`" function, which handles the lookup of everything "``link_path_walk()``" function, which handles the lookup of everything
except the final component as: except the final component as:
> Given a path (`name`) and a nameidata structure (`nd`), check that the Given a path (``name``) and a nameidata structure (``nd``), check that the
> current directory has execute permission and then advance `name` current directory has execute permission and then advance ``name``
> over one component while updating `last_type` and `last`. If that over one component while updating ``last_type`` and ``last``. If that
> was the final component, then return, otherwise call was the final component, then return, otherwise call
> `walk_component()` and repeat from the top. ``walk_component()`` and repeat from the top.
`walk_component()` is even easier. If the component is `LAST_DOTS`, ``walk_component()`` is even easier. If the component is ``LAST_DOTS``,
it calls `handle_dots()` which does the necessary locking as already it calls ``handle_dots()`` which does the necessary locking as already
described. If it finds a `LAST_NORM` component it first calls described. If it finds a ``LAST_NORM`` component it first calls
"`lookup_fast()`" which only looks in the dcache, but will ask the "``lookup_fast()``" which only looks in the dcache, but will ask the
filesystem to revalidate the result if it is that sort of filesystem. filesystem to revalidate the result if it is that sort of filesystem.
If that doesn't get a good result, it calls "`lookup_slow()`" which If that doesn't get a good result, it calls "``lookup_slow()``" which
takes `i_rwsem`, rechecks the cache, and then asks the filesystem takes ``i_rwsem``, rechecks the cache, and then asks the filesystem
to find a definitive answer. Each of these will call to find a definitive answer. Each of these will call
`follow_managed()` (as described below) to handle any mount points. ``follow_managed()`` (as described below) to handle any mount points.
In the absence of symbolic links, `walk_component()` creates a new In the absence of symbolic links, ``walk_component()`` creates a new
`struct path` containing a counted reference to the new dentry and a ``struct path`` containing a counted reference to the new dentry and a
reference to the new `vfsmount` which is only counted if it is reference to the new ``vfsmount`` which is only counted if it is
different from the previous `vfsmount`. It then calls different from the previous ``vfsmount``. It then calls
`path_to_nameidata()` to install the new `struct path` in the ``path_to_nameidata()`` to install the new ``struct path`` in the
`struct nameidata` and drop the unneeded references. ``struct nameidata`` and drop the unneeded references.
This "hand-over-hand" sequencing of getting a reference to the new This "hand-over-hand" sequencing of getting a reference to the new
dentry before dropping the reference to the previous dentry may dentry before dropping the reference to the previous dentry may
seem obvious, but is worth pointing out so that we will recognize its seem obvious, but is worth pointing out so that we will recognize its
analogue in the "RCU-walk" version. analogue in the "RCU-walk" version.
Handling the final component. Handling the final component
----------------------------- ----------------------------
`link_path_walk()` only walks as far as setting `nd->last` and ``link_path_walk()`` only walks as far as setting ``nd->last`` and
`nd->last_type` to refer to the final component of the path. It does ``nd->last_type`` to refer to the final component of the path. It does
not call `walk_component()` that last time. Handling that final not call ``walk_component()`` that last time. Handling that final
component remains for the caller to sort out. Those callers are component remains for the caller to sort out. Those callers are
`path_lookupat()`, `path_parentat()`, `path_mountpoint()` and ``path_lookupat()``, ``path_parentat()``, ``path_mountpoint()`` and
`path_openat()` each of which handles the differing requirements of ``path_openat()`` each of which handles the differing requirements of
different system calls. different system calls.
`path_parentat()` is clearly the simplest - it just wraps a little bit ``path_parentat()`` is clearly the simplest - it just wraps a little bit
of housekeeping around `link_path_walk()` and returns the parent of housekeeping around ``link_path_walk()`` and returns the parent
directory and final component to the caller. The caller will be either directory and final component to the caller. The caller will be either
aiming to create a name (via `filename_create()`) or remove or rename aiming to create a name (via ``filename_create()``) or remove or rename
a name (in which case `user_path_parent()` is used). They will use a name (in which case ``user_path_parent()`` is used). They will use
`i_rwsem` to exclude other changes while they validate and then ``i_rwsem`` to exclude other changes while they validate and then
perform their operation. perform their operation.
`path_lookupat()` is nearly as simple - it is used when an existing ``path_lookupat()`` is nearly as simple - it is used when an existing
object is wanted such as by `stat()` or `chmod()`. It essentially just object is wanted such as by ``stat()`` or ``chmod()``. It essentially just
calls `walk_component()` on the final component through a call to calls ``walk_component()`` on the final component through a call to
`lookup_last()`. `path_lookupat()` returns just the final dentry. ``lookup_last()``. ``path_lookupat()`` returns just the final dentry.
`path_mountpoint()` handles the special case of unmounting which must ``path_mountpoint()`` handles the special case of unmounting which must
not try to revalidate the mounted filesystem. It effectively not try to revalidate the mounted filesystem. It effectively
contains, through a call to `mountpoint_last()`, an alternate contains, through a call to ``mountpoint_last()``, an alternate
implementation of `lookup_slow()` which skips that step. This is implementation of ``lookup_slow()`` which skips that step. This is
important when unmounting a filesystem that is inaccessible, such as important when unmounting a filesystem that is inaccessible, such as
one provided by a dead NFS server. one provided by a dead NFS server.
Finally `path_openat()` is used for the `open()` system call; it Finally ``path_openat()`` is used for the ``open()`` system call; it
contains, in support functions starting with "`do_last()`", all the contains, in support functions starting with "``do_last()``", all the
complexity needed to handle the different subtleties of O_CREAT (with complexity needed to handle the different subtleties of O_CREAT (with
or without O_EXCL), final "`/`" characters, and trailing symbolic or without O_EXCL), final "``/``" characters, and trailing symbolic
links. We will revisit this in the final part of this series, which links. We will revisit this in the final part of this series, which
focuses on those symbolic links. "`do_last()`" will sometimes, but focuses on those symbolic links. "``do_last()``" will sometimes, but
not always, take `i_rwsem`, depending on what it finds. not always, take ``i_rwsem``, depending on what it finds.
Each of these, or the functions which call them, need to be alert to Each of these, or the functions which call them, need to be alert to
the possibility that the final component is not `LAST_NORM`. If the the possibility that the final component is not ``LAST_NORM``. If the
goal of the lookup is to create something, then any value for goal of the lookup is to create something, then any value for
`last_type` other than `LAST_NORM` will result in an error. For ``last_type`` other than ``LAST_NORM`` will result in an error. For
example if `path_parentat()` reports `LAST_DOTDOT`, then the caller example if ``path_parentat()`` reports ``LAST_DOTDOT``, then the caller
won't try to create that name. They also check for trailing slashes won't try to create that name. They also check for trailing slashes
by testing `last.name[last.len]`. If there is any character beyond by testing ``last.name[last.len]``. If there is any character beyond
the final component, it must be a trailing slash. the final component, it must be a trailing slash.
Revalidation and automounts Revalidation and automounts
...@@ -495,12 +503,12 @@ process not yet covered. One is the handling of stale cache entries ...@@ -495,12 +503,12 @@ process not yet covered. One is the handling of stale cache entries
and the other is automounts. and the other is automounts.
On filesystems that require it, the lookup routines will call the On filesystems that require it, the lookup routines will call the
`->d_revalidate()` dentry method to ensure that the cached information ``->d_revalidate()`` dentry method to ensure that the cached information
is current. This will often confirm validity or update a few details is current. This will often confirm validity or update a few details
from a server. In some cases it may find that there has been change from a server. In some cases it may find that there has been change
further up the path and that something that was thought to be valid further up the path and that something that was thought to be valid
previously isn't really. When this happens the lookup of the whole previously isn't really. When this happens the lookup of the whole
path is aborted and retried with the "`LOOKUP_REVAL`" flag set. This path is aborted and retried with the "``LOOKUP_REVAL``" flag set. This
forces revalidation to be more thorough. We will see more details of forces revalidation to be more thorough. We will see more details of
this retry process in the next article. this retry process in the next article.
...@@ -512,52 +520,55 @@ tree, but a few notes specifically related to path lookup are in order ...@@ -512,52 +520,55 @@ tree, but a few notes specifically related to path lookup are in order
here. here.
The Linux VFS has a concept of "managed" dentries which is reflected The Linux VFS has a concept of "managed" dentries which is reflected
in function names such as "`follow_managed()`". There are three in function names such as "``follow_managed()``". There are three
potentially interesting things about these dentries corresponding potentially interesting things about these dentries corresponding
to three different flags that might be set in `dentry->d_flags`: to three different flags that might be set in ``dentry->d_flags``:
### `DCACHE_MANAGE_TRANSIT` ### ``DCACHE_MANAGE_TRANSIT``
~~~~~~~~~~~~~~~~~~~~~~~
If this flag has been set, then the filesystem has requested that the If this flag has been set, then the filesystem has requested that the
`d_manage()` dentry operation be called before handling any possible ``d_manage()`` dentry operation be called before handling any possible
mount point. This can perform two particular services: mount point. This can perform two particular services:
It can block to avoid races. If an automount point is being It can block to avoid races. If an automount point is being
unmounted, the `d_manage()` function will usually wait for that unmounted, the ``d_manage()`` function will usually wait for that
process to complete before letting the new lookup proceed and possibly process to complete before letting the new lookup proceed and possibly
trigger a new automount. trigger a new automount.
It can selectively allow only some processes to transit through a It can selectively allow only some processes to transit through a
mount point. When a server process is managing automounts, it may mount point. When a server process is managing automounts, it may
need to access a directory without triggering normal automount need to access a directory without triggering normal automount
processing. That server process can identify itself to the `autofs` processing. That server process can identify itself to the ``autofs``
filesystem, which will then give it a special pass through filesystem, which will then give it a special pass through
`d_manage()` by returning `-EISDIR`. ``d_manage()`` by returning ``-EISDIR``.
### `DCACHE_MOUNTED` ### ``DCACHE_MOUNTED``
~~~~~~~~~~~~~~~~
This flag is set on every dentry that is mounted on. As Linux This flag is set on every dentry that is mounted on. As Linux
supports multiple filesystem namespaces, it is possible that the supports multiple filesystem namespaces, it is possible that the
dentry may not be mounted on in *this* namespace, just in some dentry may not be mounted on in *this* namespace, just in some
other. So this flag is seen as a hint, not a promise. other. So this flag is seen as a hint, not a promise.
If this flag is set, and `d_manage()` didn't return `-EISDIR`, If this flag is set, and ``d_manage()`` didn't return ``-EISDIR``,
`lookup_mnt()` is called to examine the mount hash table (honoring the ``lookup_mnt()`` is called to examine the mount hash table (honoring the
`mount_lock` described earlier) and possibly return a new `vfsmount` ``mount_lock`` described earlier) and possibly return a new ``vfsmount``
and a new `dentry` (both with counted references). and a new ``dentry`` (both with counted references).
### `DCACHE_NEED_AUTOMOUNT` ### ``DCACHE_NEED_AUTOMOUNT``
~~~~~~~~~~~~~~~~~~~~~~~
If `d_manage()` allowed us to get this far, and `lookup_mnt()` didn't If ``d_manage()`` allowed us to get this far, and ``lookup_mnt()`` didn't
find a mount point, then this flag causes the `d_automount()` dentry find a mount point, then this flag causes the ``d_automount()`` dentry
operation to be called. operation to be called.
The `d_automount()` operation can be arbitrarily complex and may The ``d_automount()`` operation can be arbitrarily complex and may
communicate with server processes etc. but it should ultimately either communicate with server processes etc. but it should ultimately either
report that there was an error, that there was nothing to mount, or report that there was an error, that there was nothing to mount, or
should provide an updated `struct path` with new `dentry` and `vfsmount`. should provide an updated ``struct path`` with new ``dentry`` and ``vfsmount``.
In the latter case, `finish_automount()` will be called to safely In the latter case, ``finish_automount()`` will be called to safely
install the new mount point into the mount table. install the new mount point into the mount table.
There is no new locking of import here and it is important that no There is no new locking of import here and it is important that no
...@@ -614,7 +625,7 @@ isn't in the cache, then it tries to stop gracefully and switch to ...@@ -614,7 +625,7 @@ isn't in the cache, then it tries to stop gracefully and switch to
REF-walk. REF-walk.
This stopping requires getting a counted reference on the current This stopping requires getting a counted reference on the current
`vfsmount` and `dentry`, and ensuring that these are still valid - ``vfsmount`` and ``dentry``, and ensuring that these are still valid -
that a path walk with REF-walk would have found the same entries. that a path walk with REF-walk would have found the same entries.
This is an invariant that RCU-walk must guarantee. It can only make This is an invariant that RCU-walk must guarantee. It can only make
decisions, such as selecting the next step, that are decisions which decisions, such as selecting the next step, that are decisions which
...@@ -625,21 +636,21 @@ RCU-walk finds it cannot stop gracefully, it simply gives up and ...@@ -625,21 +636,21 @@ RCU-walk finds it cannot stop gracefully, it simply gives up and
restarts from the top with REF-walk. restarts from the top with REF-walk.
This pattern of "try RCU-walk, if that fails try REF-walk" can be This pattern of "try RCU-walk, if that fails try REF-walk" can be
clearly seen in functions like `filename_lookup()`, clearly seen in functions like ``filename_lookup()``,
`filename_parentat()`, `filename_mountpoint()`, ``filename_parentat()``, ``filename_mountpoint()``,
`do_filp_open()`, and `do_file_open_root()`. These five ``do_filp_open()``, and ``do_file_open_root()``. These five
correspond roughly to the four `path_`* functions we met earlier, correspond roughly to the four ``path_``* functions we met earlier,
each of which calls `link_path_walk()`. The `path_*` functions are each of which calls ``link_path_walk()``. The ``path_*`` functions are
called using different mode flags until a mode is found which works. called using different mode flags until a mode is found which works.
They are first called with `LOOKUP_RCU` set to request "RCU-walk". If They are first called with ``LOOKUP_RCU`` set to request "RCU-walk". If
that fails with the error `ECHILD` they are called again with no that fails with the error ``ECHILD`` they are called again with no
special flag to request "REF-walk". If either of those report the special flag to request "REF-walk". If either of those report the
error `ESTALE` a final attempt is made with `LOOKUP_REVAL` set (and no error ``ESTALE`` a final attempt is made with ``LOOKUP_REVAL`` set (and no
`LOOKUP_RCU`) to ensure that entries found in the cache are forcibly ``LOOKUP_RCU``) to ensure that entries found in the cache are forcibly
revalidated - normally entries are only revalidated if the filesystem revalidated - normally entries are only revalidated if the filesystem
determines that they are too old to trust. determines that they are too old to trust.
The `LOOKUP_RCU` attempt may drop that flag internally and switch to The ``LOOKUP_RCU`` attempt may drop that flag internally and switch to
REF-walk, but will never then try to switch back to RCU-walk. Places REF-walk, but will never then try to switch back to RCU-walk. Places
that trip up RCU-walk are much more likely to be near the leaves and that trip up RCU-walk are much more likely to be near the leaves and
so it is very unlikely that there will be much, if any, benefit from so it is very unlikely that there will be much, if any, benefit from
...@@ -649,7 +660,7 @@ RCU and seqlocks: fast and light ...@@ -649,7 +660,7 @@ RCU and seqlocks: fast and light
-------------------------------- --------------------------------
RCU is, unsurprisingly, critical to RCU-walk mode. The RCU is, unsurprisingly, critical to RCU-walk mode. The
`rcu_read_lock()` is held for the entire time that RCU-walk is walking ``rcu_read_lock()`` is held for the entire time that RCU-walk is walking
down a path. The particular guarantee it provides is that the key down a path. The particular guarantee it provides is that the key
data structures - dentries, inodes, super_blocks, and mounts - will data structures - dentries, inodes, super_blocks, and mounts - will
not be freed while the lock is held. They might be unlinked or not be freed while the lock is held. They might be unlinked or
...@@ -661,7 +672,7 @@ seqlocks. ...@@ -661,7 +672,7 @@ seqlocks.
As we saw above, REF-walk holds a counted reference to the current As we saw above, REF-walk holds a counted reference to the current
dentry and the current vfsmount, and does not release those references dentry and the current vfsmount, and does not release those references
before taking references to the "next" dentry or vfsmount. It also before taking references to the "next" dentry or vfsmount. It also
sometimes takes the `d_lock` spinlock. These references and locks are sometimes takes the ``d_lock`` spinlock. These references and locks are
taken to prevent certain changes from happening. RCU-walk must not taken to prevent certain changes from happening. RCU-walk must not
take those references or locks and so cannot prevent such changes. take those references or locks and so cannot prevent such changes.
Instead, it checks to see if a change has been made, and aborts or Instead, it checks to see if a change has been made, and aborts or
...@@ -671,123 +682,126 @@ To preserve the invariant mentioned above (that RCU-walk may only make ...@@ -671,123 +682,126 @@ To preserve the invariant mentioned above (that RCU-walk may only make
decisions that REF-walk could have made), it must make the checks at decisions that REF-walk could have made), it must make the checks at
or near the same places that REF-walk holds the references. So, when or near the same places that REF-walk holds the references. So, when
REF-walk increments a reference count or takes a spinlock, RCU-walk REF-walk increments a reference count or takes a spinlock, RCU-walk
samples the status of a seqlock using `read_seqcount_begin()` or a samples the status of a seqlock using ``read_seqcount_begin()`` or a
similar function. When REF-walk decrements the count or drops the similar function. When REF-walk decrements the count or drops the
lock, RCU-walk checks if the sampled status is still valid using lock, RCU-walk checks if the sampled status is still valid using
`read_seqcount_retry()` or similar. ``read_seqcount_retry()`` or similar.
However, there is a little bit more to seqlocks than that. If However, there is a little bit more to seqlocks than that. If
RCU-walk accesses two different fields in a seqlock-protected RCU-walk accesses two different fields in a seqlock-protected
structure, or accesses the same field twice, there is no a priori structure, or accesses the same field twice, there is no a priori
guarantee of any consistency between those accesses. When consistency guarantee of any consistency between those accesses. When consistency
is needed - which it usually is - RCU-walk must take a copy and then is needed - which it usually is - RCU-walk must take a copy and then
use `read_seqcount_retry()` to validate that copy. use ``read_seqcount_retry()`` to validate that copy.
`read_seqcount_retry()` not only checks the sequence number, but also ``read_seqcount_retry()`` not only checks the sequence number, but also
imposes a memory barrier so that no memory-read instruction from imposes a memory barrier so that no memory-read instruction from
*before* the call can be delayed until *after* the call, either by the *before* the call can be delayed until *after* the call, either by the
CPU or by the compiler. A simple example of this can be seen in CPU or by the compiler. A simple example of this can be seen in
`slow_dentry_cmp()` which, for filesystems which do not use simple ``slow_dentry_cmp()`` which, for filesystems which do not use simple
byte-wise name equality, calls into the filesystem to compare a name byte-wise name equality, calls into the filesystem to compare a name
against a dentry. The length and name pointer are copied into local against a dentry. The length and name pointer are copied into local
variables, then `read_seqcount_retry()` is called to confirm the two variables, then ``read_seqcount_retry()`` is called to confirm the two
are consistent, and only then is `->d_compare()` called. When are consistent, and only then is ``->d_compare()`` called. When
standard filename comparison is used, `dentry_cmp()` is called standard filename comparison is used, ``dentry_cmp()`` is called
instead. Notably it does _not_ use `read_seqcount_retry()`, but instead. Notably it does _not_ use ``read_seqcount_retry()``, but
instead has a large comment explaining why the consistency guarantee instead has a large comment explaining why the consistency guarantee
isn't necessary. A subsequent `read_seqcount_retry()` will be isn't necessary. A subsequent ``read_seqcount_retry()`` will be
sufficient to catch any problem that could occur at this point. sufficient to catch any problem that could occur at this point.
With that little refresher on seqlocks out of the way we can look at With that little refresher on seqlocks out of the way we can look at
the bigger picture of how RCU-walk uses seqlocks. the bigger picture of how RCU-walk uses seqlocks.
### `mount_lock` and `nd->m_seq` ### ``mount_lock`` and ``nd->m_seq``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We already met the `mount_lock` seqlock when REF-walk used it to We already met the ``mount_lock`` seqlock when REF-walk used it to
ensure that crossing a mount point is performed safely. RCU-walk uses ensure that crossing a mount point is performed safely. RCU-walk uses
it for that too, but for quite a bit more. it for that too, but for quite a bit more.
Instead of taking a counted reference to each `vfsmount` as it Instead of taking a counted reference to each ``vfsmount`` as it
descends the tree, RCU-walk samples the state of `mount_lock` at the descends the tree, RCU-walk samples the state of ``mount_lock`` at the
start of the walk and stores this initial sequence number in the start of the walk and stores this initial sequence number in the
`struct nameidata` in the `m_seq` field. This one lock and one ``struct nameidata`` in the ``m_seq`` field. This one lock and one
sequence number are used to validate all accesses to all `vfsmounts`, sequence number are used to validate all accesses to all ``vfsmounts``,
and all mount point crossings. As changes to the mount table are and all mount point crossings. As changes to the mount table are
relatively rare, it is reasonable to fall back on REF-walk any time relatively rare, it is reasonable to fall back on REF-walk any time
that any "mount" or "unmount" happens. that any "mount" or "unmount" happens.
`m_seq` is checked (using `read_seqretry()`) at the end of an RCU-walk ``m_seq`` is checked (using ``read_seqretry()``) at the end of an RCU-walk
sequence, whether switching to REF-walk for the rest of the path or sequence, whether switching to REF-walk for the rest of the path or
when the end of the path is reached. It is also checked when stepping when the end of the path is reached. It is also checked when stepping
down over a mount point (in `__follow_mount_rcu()`) or up (in down over a mount point (in ``__follow_mount_rcu()``) or up (in
`follow_dotdot_rcu()`). If it is ever found to have changed, the ``follow_dotdot_rcu()``). If it is ever found to have changed, the
whole RCU-walk sequence is aborted and the path is processed again by whole RCU-walk sequence is aborted and the path is processed again by
REF-walk. REF-walk.
If RCU-walk finds that `mount_lock` hasn't changed then it can be sure If RCU-walk finds that ``mount_lock`` hasn't changed then it can be sure
that, had REF-walk taken counted references on each vfsmount, the that, had REF-walk taken counted references on each vfsmount, the
results would have been the same. This ensures the invariant holds, results would have been the same. This ensures the invariant holds,
at least for vfsmount structures. at least for vfsmount structures.
### `dentry->d_seq` and `nd->seq`. ### ``dentry->d_seq`` and ``nd->seq``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In place of taking a count or lock on `d_reflock`, RCU-walk samples In place of taking a count or lock on ``d_reflock``, RCU-walk samples
the per-dentry `d_seq` seqlock, and stores the sequence number in the the per-dentry ``d_seq`` seqlock, and stores the sequence number in the
`seq` field of the nameidata structure, so `nd->seq` should always be ``seq`` field of the nameidata structure, so ``nd->seq`` should always be
the current sequence number of `nd->dentry`. This number needs to be the current sequence number of ``nd->dentry``. This number needs to be
revalidated after copying, and before using, the name, parent, or revalidated after copying, and before using, the name, parent, or
inode of the dentry. inode of the dentry.
The handling of the name we have already looked at, and the parent is The handling of the name we have already looked at, and the parent is
only accessed in `follow_dotdot_rcu()` which fairly trivially follows only accessed in ``follow_dotdot_rcu()`` which fairly trivially follows
the required pattern, though it does so for three different cases. the required pattern, though it does so for three different cases.
When not at a mount point, `d_parent` is followed and its `d_seq` is When not at a mount point, ``d_parent`` is followed and its ``d_seq`` is
collected. When we are at a mount point, we instead follow the collected. When we are at a mount point, we instead follow the
`mnt->mnt_mountpoint` link to get a new dentry and collect its ``mnt->mnt_mountpoint`` link to get a new dentry and collect its
`d_seq`. Then, after finally finding a `d_parent` to follow, we must ``d_seq``. Then, after finally finding a ``d_parent`` to follow, we must
check if we have landed on a mount point and, if so, must find that check if we have landed on a mount point and, if so, must find that
mount point and follow the `mnt->mnt_root` link. This would imply a mount point and follow the ``mnt->mnt_root`` link. This would imply a
somewhat unusual, but certainly possible, circumstance where the somewhat unusual, but certainly possible, circumstance where the
starting point of the path lookup was in part of the filesystem that starting point of the path lookup was in part of the filesystem that
was mounted on, and so not visible from the root. was mounted on, and so not visible from the root.
The inode pointer, stored in `->d_inode`, is a little more The inode pointer, stored in ``->d_inode``, is a little more
interesting. The inode will always need to be accessed at least interesting. The inode will always need to be accessed at least
twice, once to determine if it is NULL and once to verify access twice, once to determine if it is NULL and once to verify access
permissions. Symlink handling requires a validated inode pointer too. permissions. Symlink handling requires a validated inode pointer too.
Rather than revalidating on each access, a copy is made on the first Rather than revalidating on each access, a copy is made on the first
access and it is stored in the `inode` field of `nameidata` from where access and it is stored in the ``inode`` field of ``nameidata`` from where
it can be safely accessed without further validation. it can be safely accessed without further validation.
`lookup_fast()` is the only lookup routine that is used in RCU-mode, ``lookup_fast()`` is the only lookup routine that is used in RCU-mode,
`lookup_slow()` being too slow and requiring locks. It is in ``lookup_slow()`` being too slow and requiring locks. It is in
`lookup_fast()` that we find the important "hand over hand" tracking ``lookup_fast()`` that we find the important "hand over hand" tracking
of the current dentry. of the current dentry.
The current `dentry` and current `seq` number are passed to The current ``dentry`` and current ``seq`` number are passed to
`__d_lookup_rcu()` which, on success, returns a new `dentry` and a ``__d_lookup_rcu()`` which, on success, returns a new ``dentry`` and a
new `seq` number. `lookup_fast()` then copies the inode pointer and new ``seq`` number. ``lookup_fast()`` then copies the inode pointer and
revalidates the new `seq` number. It then validates the old `dentry` revalidates the new ``seq`` number. It then validates the old ``dentry``
with the old `seq` number one last time and only then continues. This with the old ``seq`` number one last time and only then continues. This
process of getting the `seq` number of the new dentry and then process of getting the ``seq`` number of the new dentry and then
checking the `seq` number of the old exactly mirrors the process of checking the ``seq`` number of the old exactly mirrors the process of
getting a counted reference to the new dentry before dropping that for getting a counted reference to the new dentry before dropping that for
the old dentry which we saw in REF-walk. the old dentry which we saw in REF-walk.
### No `inode->i_rwsem` or even `rename_lock` ### No ``inode->i_rwsem`` or even ``rename_lock``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A semaphore is a fairly heavyweight lock that can only be taken when it is A semaphore is a fairly heavyweight lock that can only be taken when it is
permissible to sleep. As `rcu_read_lock()` forbids sleeping, permissible to sleep. As ``rcu_read_lock()`` forbids sleeping,
`inode->i_rwsem` plays no role in RCU-walk. If some other thread does ``inode->i_rwsem`` plays no role in RCU-walk. If some other thread does
take `i_rwsem` and modifies the directory in a way that RCU-walk needs take ``i_rwsem`` and modifies the directory in a way that RCU-walk needs
to notice, the result will be either that RCU-walk fails to find the to notice, the result will be either that RCU-walk fails to find the
dentry that it is looking for, or it will find a dentry which dentry that it is looking for, or it will find a dentry which
`read_seqretry()` won't validate. In either case it will drop down to ``read_seqretry()`` won't validate. In either case it will drop down to
REF-walk mode which can take whatever locks are needed. REF-walk mode which can take whatever locks are needed.
Though `rename_lock` could be used by RCU-walk as it doesn't require Though ``rename_lock`` could be used by RCU-walk as it doesn't require
any sleeping, RCU-walk doesn't bother. REF-walk uses `rename_lock` to any sleeping, RCU-walk doesn't bother. REF-walk uses ``rename_lock`` to
protect against the possibility of hash chains in the dcache changing protect against the possibility of hash chains in the dcache changing
while they are being searched. This can result in failing to find while they are being searched. This can result in failing to find
something that actually is there. When RCU-walk fails to find something that actually is there. When RCU-walk fails to find
...@@ -796,57 +810,57 @@ already drops down to REF-walk and tries again with appropriate ...@@ -796,57 +810,57 @@ already drops down to REF-walk and tries again with appropriate
locking. This neatly handles all cases, so adding extra checks on locking. This neatly handles all cases, so adding extra checks on
rename_lock would bring no significant value. rename_lock would bring no significant value.
`unlazy walk()` and `complete_walk()` ``unlazy walk()`` and ``complete_walk()``
------------------------------------- -------------------------------------
That "dropping down to REF-walk" typically involves a call to That "dropping down to REF-walk" typically involves a call to
`unlazy_walk()`, so named because "RCU-walk" is also sometimes ``unlazy_walk()``, so named because "RCU-walk" is also sometimes
referred to as "lazy walk". `unlazy_walk()` is called when referred to as "lazy walk". ``unlazy_walk()`` is called when
following the path down to the current vfsmount/dentry pair seems to following the path down to the current vfsmount/dentry pair seems to
have proceeded successfully, but the next step is problematic. This have proceeded successfully, but the next step is problematic. This
can happen if the next name cannot be found in the dcache, if can happen if the next name cannot be found in the dcache, if
permission checking or name revalidation couldn't be achieved while permission checking or name revalidation couldn't be achieved while
the `rcu_read_lock()` is held (which forbids sleeping), if an the ``rcu_read_lock()`` is held (which forbids sleeping), if an
automount point is found, or in a couple of cases involving symlinks. automount point is found, or in a couple of cases involving symlinks.
It is also called from `complete_walk()` when the lookup has reached It is also called from ``complete_walk()`` when the lookup has reached
the final component, or the very end of the path, depending on which the final component, or the very end of the path, depending on which
particular flavor of lookup is used. particular flavor of lookup is used.
Other reasons for dropping out of RCU-walk that do not trigger a call Other reasons for dropping out of RCU-walk that do not trigger a call
to `unlazy_walk()` are when some inconsistency is found that cannot be to ``unlazy_walk()`` are when some inconsistency is found that cannot be
handled immediately, such as `mount_lock` or one of the `d_seq` handled immediately, such as ``mount_lock`` or one of the ``d_seq``
seqlocks reporting a change. In these cases the relevant function seqlocks reporting a change. In these cases the relevant function
will return `-ECHILD` which will percolate up until it triggers a new will return ``-ECHILD`` which will percolate up until it triggers a new
attempt from the top using REF-walk. attempt from the top using REF-walk.
For those cases where `unlazy_walk()` is an option, it essentially For those cases where ``unlazy_walk()`` is an option, it essentially
takes a reference on each of the pointers that it holds (vfsmount, takes a reference on each of the pointers that it holds (vfsmount,
dentry, and possibly some symbolic links) and then verifies that the dentry, and possibly some symbolic links) and then verifies that the
relevant seqlocks have not been changed. If there have been changes, relevant seqlocks have not been changed. If there have been changes,
it, too, aborts with `-ECHILD`, otherwise the transition to REF-walk it, too, aborts with ``-ECHILD``, otherwise the transition to REF-walk
has been a success and the lookup process continues. has been a success and the lookup process continues.
Taking a reference on those pointers is not quite as simple as just Taking a reference on those pointers is not quite as simple as just
incrementing a counter. That works to take a second reference if you incrementing a counter. That works to take a second reference if you
already have one (often indirectly through another object), but it already have one (often indirectly through another object), but it
isn't sufficient if you don't actually have a counted reference at isn't sufficient if you don't actually have a counted reference at
all. For `dentry->d_lockref`, it is safe to increment the reference all. For ``dentry->d_lockref``, it is safe to increment the reference
counter to get a reference unless it has been explicitly marked as counter to get a reference unless it has been explicitly marked as
"dead" which involves setting the counter to `-128`. "dead" which involves setting the counter to ``-128``.
`lockref_get_not_dead()` achieves this. ``lockref_get_not_dead()`` achieves this.
For `mnt->mnt_count` it is safe to take a reference as long as For ``mnt->mnt_count`` it is safe to take a reference as long as
`mount_lock` is then used to validate the reference. If that ``mount_lock`` is then used to validate the reference. If that
validation fails, it may *not* be safe to just drop that reference in validation fails, it may *not* be safe to just drop that reference in
the standard way of calling `mnt_put()` - an unmount may have the standard way of calling ``mnt_put()`` - an unmount may have
progressed too far. So the code in `legitimize_mnt()`, when it progressed too far. So the code in ``legitimize_mnt()``, when it
finds that the reference it got might not be safe, checks the finds that the reference it got might not be safe, checks the
`MNT_SYNC_UMOUNT` flag to determine if a simple `mnt_put()` is ``MNT_SYNC_UMOUNT`` flag to determine if a simple ``mnt_put()`` is
correct, or if it should just decrement the count and pretend none of correct, or if it should just decrement the count and pretend none of
this ever happened. this ever happened.
Taking care in filesystems Taking care in filesystems
--------------------------- --------------------------
RCU-walk depends almost entirely on cached information and often will RCU-walk depends almost entirely on cached information and often will
not call into the filesystem at all. However there are two places, not call into the filesystem at all. However there are two places,
...@@ -856,26 +870,26 @@ careful. ...@@ -856,26 +870,26 @@ careful.
If the filesystem has non-standard permission-checking requirements - If the filesystem has non-standard permission-checking requirements -
such as a networked filesystem which may need to check with the server such as a networked filesystem which may need to check with the server
- the `i_op->permission` interface might be called during RCU-walk. - the ``i_op->permission`` interface might be called during RCU-walk.
In this case an extra "`MAY_NOT_BLOCK`" flag is passed so that it In this case an extra "``MAY_NOT_BLOCK``" flag is passed so that it
knows not to sleep, but to return `-ECHILD` if it cannot complete knows not to sleep, but to return ``-ECHILD`` if it cannot complete
promptly. `i_op->permission` is given the inode pointer, not the promptly. ``i_op->permission`` is given the inode pointer, not the
dentry, so it doesn't need to worry about further consistency checks. dentry, so it doesn't need to worry about further consistency checks.
However if it accesses any other filesystem data structures, it must However if it accesses any other filesystem data structures, it must
ensure they are safe to be accessed with only the `rcu_read_lock()` ensure they are safe to be accessed with only the ``rcu_read_lock()``
held. This typically means they must be freed using `kfree_rcu()` or held. This typically means they must be freed using ``kfree_rcu()`` or
similar. similar.
[`READ_ONCE()`]: https://lwn.net/Articles/624126/ .. _READ_ONCE: https://lwn.net/Articles/624126/
If the filesystem may need to revalidate dcache entries, then If the filesystem may need to revalidate dcache entries, then
`d_op->d_revalidate` may be called in RCU-walk too. This interface ``d_op->d_revalidate`` may be called in RCU-walk too. This interface
*is* passed the dentry but does not have access to the `inode` or the *is* passed the dentry but does not have access to the ``inode`` or the
`seq` number from the `nameidata`, so it needs to be extra careful ``seq`` number from the ``nameidata``, so it needs to be extra careful
when accessing fields in the dentry. This "extra care" typically when accessing fields in the dentry. This "extra care" typically
involves using [`READ_ONCE()`] to access fields, and verifying the involves using `READ_ONCE() <READ_ONCE_>`_ to access fields, and verifying the
result is not NULL before using it. This pattern can be seen in result is not NULL before using it. This pattern can be seen in
`nfs_lookup_revalidate()`. ``nfs_lookup_revalidate()``.
A pair of patterns A pair of patterns
------------------ ------------------
...@@ -886,14 +900,14 @@ being aware of. ...@@ -886,14 +900,14 @@ being aware of.
The first is "try quickly and check, if that fails try slowly". We The first is "try quickly and check, if that fails try slowly". We
can see that in the high-level approach of first trying RCU-walk and can see that in the high-level approach of first trying RCU-walk and
then trying REF-walk, and in places where `unlazy_walk()` is used to then trying REF-walk, and in places where ``unlazy_walk()`` is used to
switch to REF-walk for the rest of the path. We also saw it earlier switch to REF-walk for the rest of the path. We also saw it earlier
in `dget_parent()` when following a "`..`" link. It tries a quick way in ``dget_parent()`` when following a "``..``" link. It tries a quick way
to get a reference, then falls back to taking locks if needed. to get a reference, then falls back to taking locks if needed.
The second pattern is "try quickly and check, if that fails try The second pattern is "try quickly and check, if that fails try
again - repeatedly". This is seen with the use of `rename_lock` and again - repeatedly". This is seen with the use of ``rename_lock`` and
`mount_lock` in REF-walk. RCU-walk doesn't make use of this pattern - ``mount_lock`` in REF-walk. RCU-walk doesn't make use of this pattern -
if anything goes wrong it is much safer to just abort and try a more if anything goes wrong it is much safer to just abort and try a more
sedate approach. sedate approach.
...@@ -929,8 +943,8 @@ Conceptually, symbolic links could be handled by editing the path. If ...@@ -929,8 +943,8 @@ Conceptually, symbolic links could be handled by editing the path. If
a component name refers to a symbolic link, then that component is a component name refers to a symbolic link, then that component is
replaced by the body of the link and, if that body starts with a '/', replaced by the body of the link and, if that body starts with a '/',
then all preceding parts of the path are discarded. This is what the then all preceding parts of the path are discarded. This is what the
"`readlink -f`" command does, though it also edits out "`.`" and "``readlink -f``" command does, though it also edits out "``.``" and
"`..`" components. "``..``" components.
Directly editing the path string is not really necessary when looking Directly editing the path string is not really necessary when looking
up a path, and discarding early components is pointless as they aren't up a path, and discarding early components is pointless as they aren't
...@@ -946,19 +960,19 @@ There are two reasons for placing limits on how many symlinks can ...@@ -946,19 +960,19 @@ There are two reasons for placing limits on how many symlinks can
occur in a single path lookup. The most obvious is to avoid loops. occur in a single path lookup. The most obvious is to avoid loops.
If a symlink referred to itself either directly or through If a symlink referred to itself either directly or through
intermediaries, then following the symlink can never complete intermediaries, then following the symlink can never complete
successfully - the error `ELOOP` must be returned. Loops can be successfully - the error ``ELOOP`` must be returned. Loops can be
detected without imposing limits, but limits are the simplest solution detected without imposing limits, but limits are the simplest solution
and, given the second reason for restriction, quite sufficient. and, given the second reason for restriction, quite sufficient.
[outlined recently]: http://thread.gmane.org/gmane.linux.kernel/1934390/focus=1934550 .. _outlined recently: http://thread.gmane.org/gmane.linux.kernel/1934390/focus=1934550
The second reason was [outlined recently] by Linus: The second reason was `outlined recently`_ by Linus:
> Because it's a latency and DoS issue too. We need to react well to Because it's a latency and DoS issue too. We need to react well to
> true loops, but also to "very deep" non-loops. It's not about memory true loops, but also to "very deep" non-loops. It's not about memory
> use, it's about users triggering unreasonable CPU resources. use, it's about users triggering unreasonable CPU resources.
Linux imposes a limit on the length of any pathname: `PATH_MAX`, which Linux imposes a limit on the length of any pathname: ``PATH_MAX``, which
is 4096. There are a number of reasons for this limit; not letting the is 4096. There are a number of reasons for this limit; not letting the
kernel spend too much time on just one path is one of them. With kernel spend too much time on just one path is one of them. With
symbolic links you can effectively generate much longer paths so some symbolic links you can effectively generate much longer paths so some
...@@ -968,7 +982,7 @@ further limit of eight on the maximum depth of recursion, but that was ...@@ -968,7 +982,7 @@ further limit of eight on the maximum depth of recursion, but that was
raised to 40 when a separate stack was implemented, so there is now raised to 40 when a separate stack was implemented, so there is now
just the one limit. just the one limit.
The `nameidata` structure that we met in an earlier article contains a The ``nameidata`` structure that we met in an earlier article contains a
small stack that can be used to store the remaining part of up to two small stack that can be used to store the remaining part of up to two
symlinks. In many cases this will be sufficient. If it isn't, a symlinks. In many cases this will be sufficient. If it isn't, a
separate stack is allocated with room for 40 symlinks. Pathname separate stack is allocated with room for 40 symlinks. Pathname
...@@ -988,13 +1002,13 @@ to external storage. It is particularly important for RCU-walk to be ...@@ -988,13 +1002,13 @@ to external storage. It is particularly important for RCU-walk to be
able to find and temporarily hold onto these cached entries, so that able to find and temporarily hold onto these cached entries, so that
it doesn't need to drop down into REF-walk. it doesn't need to drop down into REF-walk.
[object-oriented design pattern]: https://lwn.net/Articles/446317/ .. _object-oriented design pattern: https://lwn.net/Articles/446317/
While each filesystem is free to make its own choice, symlinks are While each filesystem is free to make its own choice, symlinks are
typically stored in one of two places. Short symlinks are often typically stored in one of two places. Short symlinks are often
stored directly in the inode. When a filesystem allocates a `struct stored directly in the inode. When a filesystem allocates a ``struct
inode` it typically allocates extra space to store private data (a inode`` it typically allocates extra space to store private data (a
common [object-oriented design pattern] in the kernel). This will common `object-oriented design pattern`_ in the kernel). This will
sometimes include space for a symlink. The other common location is sometimes include space for a symlink. The other common location is
in the page cache, which normally stores the content of files. The in the page cache, which normally stores the content of files. The
pathname in a symlink can be seen as the content of that symlink and pathname in a symlink can be seen as the content of that symlink and
...@@ -1009,13 +1023,13 @@ the inode which, itself, is protected by RCU or by a counted reference ...@@ -1009,13 +1023,13 @@ the inode which, itself, is protected by RCU or by a counted reference
on the dentry. This means that the mechanisms that pathname lookup on the dentry. This means that the mechanisms that pathname lookup
uses to access the dcache and icache (inode cache) safely are quite uses to access the dcache and icache (inode cache) safely are quite
sufficient for accessing some cached symlinks safely. In these cases, sufficient for accessing some cached symlinks safely. In these cases,
the `i_link` pointer in the inode is set to point to wherever the the ``i_link`` pointer in the inode is set to point to wherever the
symlink is stored and it can be accessed directly whenever needed. symlink is stored and it can be accessed directly whenever needed.
When the symlink is stored in the page cache or elsewhere, the When the symlink is stored in the page cache or elsewhere, the
situation is not so straightforward. A reference on a dentry or even situation is not so straightforward. A reference on a dentry or even
on an inode does not imply any reference on cached pages of that on an inode does not imply any reference on cached pages of that
inode, and even an `rcu_read_lock()` is not sufficient to ensure that inode, and even an ``rcu_read_lock()`` is not sufficient to ensure that
a page will not disappear. So for these symlinks the pathname lookup a page will not disappear. So for these symlinks the pathname lookup
code needs to ask the filesystem to provide a stable reference and, code needs to ask the filesystem to provide a stable reference and,
significantly, needs to release that reference when it is finished significantly, needs to release that reference when it is finished
...@@ -1025,48 +1039,48 @@ Taking a reference to a cache page is often possible even in RCU-walk ...@@ -1025,48 +1039,48 @@ Taking a reference to a cache page is often possible even in RCU-walk
mode. It does require making changes to memory, which is best avoided, mode. It does require making changes to memory, which is best avoided,
but that isn't necessarily a big cost and it is better than dropping but that isn't necessarily a big cost and it is better than dropping
out of RCU-walk mode completely. Even filesystems that allocate out of RCU-walk mode completely. Even filesystems that allocate
space to copy the symlink into can use `GFP_ATOMIC` to often successfully space to copy the symlink into can use ``GFP_ATOMIC`` to often successfully
allocate memory without the need to drop out of RCU-walk. If a allocate memory without the need to drop out of RCU-walk. If a
filesystem cannot successfully get a reference in RCU-walk mode, it filesystem cannot successfully get a reference in RCU-walk mode, it
must return `-ECHILD` and `unlazy_walk()` will be called to return to must return ``-ECHILD`` and ``unlazy_walk()`` will be called to return to
REF-walk mode in which the filesystem is allowed to sleep. REF-walk mode in which the filesystem is allowed to sleep.
The place for all this to happen is the `i_op->follow_link()` inode The place for all this to happen is the ``i_op->follow_link()`` inode
method. In the present mainline code this is never actually called in method. In the present mainline code this is never actually called in
RCU-walk mode as the rewrite is not quite complete. It is likely that RCU-walk mode as the rewrite is not quite complete. It is likely that
in a future release this method will be passed an `inode` pointer when in a future release this method will be passed an ``inode`` pointer when
called in RCU-walk mode so it both (1) knows to be careful, and (2) has the called in RCU-walk mode so it both (1) knows to be careful, and (2) has the
validated pointer. Much like the `i_op->permission()` method we validated pointer. Much like the ``i_op->permission()`` method we
looked at previously, `->follow_link()` would need to be careful that looked at previously, ``->follow_link()`` would need to be careful that
all the data structures it references are safe to be accessed while all the data structures it references are safe to be accessed while
holding no counted reference, only the RCU lock. Though getting a holding no counted reference, only the RCU lock. Though getting a
reference with `->follow_link()` is not yet done in RCU-walk mode, the reference with ``->follow_link()`` is not yet done in RCU-walk mode, the
code is ready to release the reference when that does happen. code is ready to release the reference when that does happen.
This need to drop the reference to a symlink adds significant This need to drop the reference to a symlink adds significant
complexity. It requires a reference to the inode so that the complexity. It requires a reference to the inode so that the
`i_op->put_link()` inode operation can be called. In REF-walk, that ``i_op->put_link()`` inode operation can be called. In REF-walk, that
reference is kept implicitly through a reference to the dentry, so reference is kept implicitly through a reference to the dentry, so
keeping the `struct path` of the symlink is easiest. For RCU-walk, keeping the ``struct path`` of the symlink is easiest. For RCU-walk,
the pointer to the inode is kept separately. To allow switching from the pointer to the inode is kept separately. To allow switching from
RCU-walk back to REF-walk in the middle of processing nested symlinks RCU-walk back to REF-walk in the middle of processing nested symlinks
we also need the seq number for the dentry so we can confirm that we also need the seq number for the dentry so we can confirm that
switching back was safe. switching back was safe.
Finally, when providing a reference to a symlink, the filesystem also Finally, when providing a reference to a symlink, the filesystem also
provides an opaque "cookie" that must be passed to `->put_link()` so that it provides an opaque "cookie" that must be passed to ``->put_link()`` so that it
knows what to free. This might be the allocated memory area, or a knows what to free. This might be the allocated memory area, or a
pointer to the `struct page` in the page cache, or something else pointer to the ``struct page`` in the page cache, or something else
completely. Only the filesystem knows what it is. completely. Only the filesystem knows what it is.
In order for the reference to each symlink to be dropped when the walk completes, In order for the reference to each symlink to be dropped when the walk completes,
whether in RCU-walk or REF-walk, the symlink stack needs to contain, whether in RCU-walk or REF-walk, the symlink stack needs to contain,
along with the path remnants: along with the path remnants:
- the `struct path` to provide a reference to the inode in REF-walk - the ``struct path`` to provide a reference to the inode in REF-walk
- the `struct inode *` to provide a reference to the inode in RCU-walk - the ``struct inode *`` to provide a reference to the inode in RCU-walk
- the `seq` to allow the path to be safely switched from RCU-walk to REF-walk - the ``seq`` to allow the path to be safely switched from RCU-walk to REF-walk
- the `cookie` that tells `->put_path()` what to put. - the ``cookie`` that tells ``->put_path()`` what to put.
This means that each entry in the symlink stack needs to hold five This means that each entry in the symlink stack needs to hold five
pointers and an integer instead of just one pointer (the path pointers and an integer instead of just one pointer (the path
...@@ -1075,28 +1089,28 @@ with 40 entries it adds up to 1600 bytes total, which is less than ...@@ -1075,28 +1089,28 @@ with 40 entries it adds up to 1600 bytes total, which is less than
half a page. So it might seem like a lot, but is by no means half a page. So it might seem like a lot, but is by no means
excessive. excessive.
Note that, in a given stack frame, the path remnant (`name`) is not Note that, in a given stack frame, the path remnant (``name``) is not
part of the symlink that the other fields refer to. It is the remnant part of the symlink that the other fields refer to. It is the remnant
to be followed once that symlink has been fully parsed. to be followed once that symlink has been fully parsed.
Following the symlink Following the symlink
--------------------- ---------------------
The main loop in `link_path_walk()` iterates seamlessly over all The main loop in ``link_path_walk()`` iterates seamlessly over all
components in the path and all of the non-final symlinks. As symlinks components in the path and all of the non-final symlinks. As symlinks
are processed, the `name` pointer is adjusted to point to a new are processed, the ``name`` pointer is adjusted to point to a new
symlink, or is restored from the stack, so that much of the loop symlink, or is restored from the stack, so that much of the loop
doesn't need to notice. Getting this `name` variable on and off the doesn't need to notice. Getting this ``name`` variable on and off the
stack is very straightforward; pushing and popping the references is stack is very straightforward; pushing and popping the references is
a little more complex. a little more complex.
When a symlink is found, `walk_component()` returns the value `1` When a symlink is found, ``walk_component()`` returns the value ``1``
(`0` is returned for any other sort of success, and a negative number (``0`` is returned for any other sort of success, and a negative number
is, as usual, an error indicator). This causes `get_link()` to be is, as usual, an error indicator). This causes ``get_link()`` to be
called; it then gets the link from the filesystem. Providing that called; it then gets the link from the filesystem. Providing that
operation is successful, the old path `name` is placed on the stack, operation is successful, the old path ``name`` is placed on the stack,
and the new value is used as the `name` for a while. When the end of and the new value is used as the ``name`` for a while. When the end of
the path is found (i.e. `*name` is `'\0'`) the old `name` is restored the path is found (i.e. ``*name`` is ``'\0'``) the old ``name`` is restored
off the stack and path walking continues. off the stack and path walking continues.
Pushing and popping the reference pointers (inode, cookie, etc.) is more Pushing and popping the reference pointers (inode, cookie, etc.) is more
...@@ -1107,112 +1121,113 @@ the symlink-just-found to avoid leaving empty path remnants that would ...@@ -1107,112 +1121,113 @@ the symlink-just-found to avoid leaving empty path remnants that would
just get in the way. just get in the way.
It is most convenient to push the new symlink references onto the It is most convenient to push the new symlink references onto the
stack in `walk_component()` immediately when the symlink is found; stack in ``walk_component()`` immediately when the symlink is found;
`walk_component()` is also the last piece of code that needs to look at the ``walk_component()`` is also the last piece of code that needs to look at the
old symlink as it walks that last component. So it is quite old symlink as it walks that last component. So it is quite
convenient for `walk_component()` to release the old symlink and pop convenient for ``walk_component()`` to release the old symlink and pop
the references just before pushing the reference information for the the references just before pushing the reference information for the
new symlink. It is guided in this by two flags; `WALK_GET`, which new symlink. It is guided in this by two flags; ``WALK_GET``, which
gives it permission to follow a symlink if it finds one, and gives it permission to follow a symlink if it finds one, and
`WALK_PUT`, which tells it to release the current symlink after it has been ``WALK_PUT``, which tells it to release the current symlink after it has been
followed. `WALK_PUT` is tested first, leading to a call to followed. ``WALK_PUT`` is tested first, leading to a call to
`put_link()`. `WALK_GET` is tested subsequently (by ``put_link()``. ``WALK_GET`` is tested subsequently (by
`should_follow_link()`) leading to a call to `pick_link()` which sets ``should_follow_link()``) leading to a call to ``pick_link()`` which sets
up the stack frame. up the stack frame.
### Symlinks with no final component ### Symlinks with no final component
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A pair of special-case symlinks deserve a little further explanation. A pair of special-case symlinks deserve a little further explanation.
Both result in a new `struct path` (with mount and dentry) being set Both result in a new ``struct path`` (with mount and dentry) being set
up in the `nameidata`, and result in `get_link()` returning `NULL`. up in the ``nameidata``, and result in ``get_link()`` returning ``NULL``.
The more obvious case is a symlink to "`/`". All symlinks starting The more obvious case is a symlink to "``/``". All symlinks starting
with "`/`" are detected in `get_link()` which resets the `nameidata` with "``/``" are detected in ``get_link()`` which resets the ``nameidata``
to point to the effective filesystem root. If the symlink only to point to the effective filesystem root. If the symlink only
contains "`/`" then there is nothing more to do, no components at all, contains "``/``" then there is nothing more to do, no components at all,
so `NULL` is returned to indicate that the symlink can be released and so ``NULL`` is returned to indicate that the symlink can be released and
the stack frame discarded. the stack frame discarded.
The other case involves things in `/proc` that look like symlinks but The other case involves things in ``/proc`` that look like symlinks but
aren't really. aren't really::
> $ ls -l /proc/self/fd/1 $ ls -l /proc/self/fd/1
> lrwx------ 1 neilb neilb 64 Jun 13 10:19 /proc/self/fd/1 -> /dev/pts/4 lrwx------ 1 neilb neilb 64 Jun 13 10:19 /proc/self/fd/1 -> /dev/pts/4
Every open file descriptor in any process is represented in `/proc` by Every open file descriptor in any process is represented in ``/proc`` by
something that looks like a symlink. It is really a reference to the something that looks like a symlink. It is really a reference to the
target file, not just the name of it. When you `readlink` these target file, not just the name of it. When you ``readlink`` these
objects you get a name that might refer to the same file - unless it objects you get a name that might refer to the same file - unless it
has been unlinked or mounted over. When `walk_component()` follows has been unlinked or mounted over. When ``walk_component()`` follows
one of these, the `->follow_link()` method in "procfs" doesn't return one of these, the ``->follow_link()`` method in "procfs" doesn't return
a string name, but instead calls `nd_jump_link()` which updates the a string name, but instead calls ``nd_jump_link()`` which updates the
`nameidata` in place to point to that target. `->follow_link()` then ``nameidata`` in place to point to that target. ``->follow_link()`` then
returns `NULL`. Again there is no final component and `get_link()` returns ``NULL``. Again there is no final component and ``get_link()``
reports this by leaving the `last_type` field of `nameidata` as reports this by leaving the ``last_type`` field of ``nameidata`` as
`LAST_BIND`. ``LAST_BIND``.
Following the symlink in the final component Following the symlink in the final component
-------------------------------------------- --------------------------------------------
All this leads to `link_path_walk()` walking down every component, and All this leads to ``link_path_walk()`` walking down every component, and
following all symbolic links it finds, until it reaches the final following all symbolic links it finds, until it reaches the final
component. This is just returned in the `last` field of `nameidata`. component. This is just returned in the ``last`` field of ``nameidata``.
For some callers, this is all they need; they want to create that For some callers, this is all they need; they want to create that
`last` name if it doesn't exist or give an error if it does. Other ``last`` name if it doesn't exist or give an error if it does. Other
callers will want to follow a symlink if one is found, and possibly callers will want to follow a symlink if one is found, and possibly
apply special handling to the last component of that symlink, rather apply special handling to the last component of that symlink, rather
than just the last component of the original file name. These callers than just the last component of the original file name. These callers
potentially need to call `link_path_walk()` again and again on potentially need to call ``link_path_walk()`` again and again on
successive symlinks until one is found that doesn't point to another successive symlinks until one is found that doesn't point to another
symlink. symlink.
This case is handled by the relevant caller of `link_path_walk()`, such as This case is handled by the relevant caller of ``link_path_walk()``, such as
`path_lookupat()` using a loop that calls `link_path_walk()`, and then ``path_lookupat()`` using a loop that calls ``link_path_walk()``, and then
handles the final component. If the final component is a symlink handles the final component. If the final component is a symlink
that needs to be followed, then `trailing_symlink()` is called to set that needs to be followed, then ``trailing_symlink()`` is called to set
things up properly and the loop repeats, calling `link_path_walk()` things up properly and the loop repeats, calling ``link_path_walk()``
again. This could loop as many as 40 times if the last component of again. This could loop as many as 40 times if the last component of
each symlink is another symlink. each symlink is another symlink.
The various functions that examine the final component and possibly The various functions that examine the final component and possibly
report that it is a symlink are `lookup_last()`, `mountpoint_last()` report that it is a symlink are ``lookup_last()``, ``mountpoint_last()``
and `do_last()`, each of which use the same convention as and ``do_last()``, each of which use the same convention as
`walk_component()` of returning `1` if a symlink was found that needs ``walk_component()`` of returning ``1`` if a symlink was found that needs
to be followed. to be followed.
Of these, `do_last()` is the most interesting as it is used for Of these, ``do_last()`` is the most interesting as it is used for
opening a file. Part of `do_last()` runs with `i_rwsem` held and this opening a file. Part of ``do_last()`` runs with ``i_rwsem`` held and this
part is in a separate function: `lookup_open()`. part is in a separate function: ``lookup_open()``.
Explaining `do_last()` completely is beyond the scope of this article, Explaining ``do_last()`` completely is beyond the scope of this article,
but a few highlights should help those interested in exploring the but a few highlights should help those interested in exploring the
code. code.
1. Rather than just finding the target file, `do_last()` needs to open 1. Rather than just finding the target file, ``do_last()`` needs to open
it. If the file was found in the dcache, then `vfs_open()` is used for it. If the file was found in the dcache, then ``vfs_open()`` is used for
this. If not, then `lookup_open()` will either call `atomic_open()` (if this. If not, then ``lookup_open()`` will either call ``atomic_open()`` (if
the filesystem provides it) to combine the final lookup with the open, or the filesystem provides it) to combine the final lookup with the open, or
will perform the separate `lookup_real()` and `vfs_create()` steps will perform the separate ``lookup_real()`` and ``vfs_create()`` steps
directly. In the later case the actual "open" of this newly found or directly. In the later case the actual "open" of this newly found or
created file will be performed by `vfs_open()`, just as if the name created file will be performed by ``vfs_open()``, just as if the name
were found in the dcache. were found in the dcache.
2. `vfs_open()` can fail with `-EOPENSTALE` if the cached information 2. ``vfs_open()`` can fail with ``-EOPENSTALE`` if the cached information
wasn't quite current enough. Rather than restarting the lookup from wasn't quite current enough. Rather than restarting the lookup from
the top with `LOOKUP_REVAL` set, `lookup_open()` is called instead, the top with ``LOOKUP_REVAL`` set, ``lookup_open()`` is called instead,
giving the filesystem a chance to resolve small inconsistencies. giving the filesystem a chance to resolve small inconsistencies.
If that doesn't work, only then is the lookup restarted from the top. If that doesn't work, only then is the lookup restarted from the top.
3. An open with O_CREAT **does** follow a symlink in the final component, 3. An open with O_CREAT **does** follow a symlink in the final component,
unlike other creation system calls (like `mkdir`). So the sequence: unlike other creation system calls (like ``mkdir``). So the sequence::
> ln -s bar /tmp/foo ln -s bar /tmp/foo
> echo hello > /tmp/foo echo hello > /tmp/foo
will create a file called `/tmp/bar`. This is not permitted if will create a file called ``/tmp/bar``. This is not permitted if
`O_EXCL` is set but otherwise is handled for an O_CREAT open much ``O_EXCL`` is set but otherwise is handled for an O_CREAT open much
like for a non-creating open: `should_follow_link()` returns `1`, and like for a non-creating open: ``should_follow_link()`` returns ``1``, and
so does `do_last()` so that `trailing_symlink()` gets called and the so does ``do_last()`` so that ``trailing_symlink()`` gets called and the
open process continues on the symlink that was found. open process continues on the symlink that was found.
Updating the access time Updating the access time
...@@ -1227,110 +1242,112 @@ footprints are best kept to a minimum. ...@@ -1227,110 +1242,112 @@ footprints are best kept to a minimum.
One other place where walking down a symlink can involve leaving One other place where walking down a symlink can involve leaving
footprints in a way that doesn't affect directories is in updating access times. footprints in a way that doesn't affect directories is in updating access times.
In Unix (and Linux) every filesystem object has a "last accessed In Unix (and Linux) every filesystem object has a "last accessed
time", or "`atime`". Passing through a directory to access a file time", or "``atime``". Passing through a directory to access a file
within is not considered to be an access for the purposes of within is not considered to be an access for the purposes of
`atime`; only listing the contents of a directory can update its `atime`. ``atime``; only listing the contents of a directory can update its ``atime``.
Symlinks are different it seems. Both reading a symlink (with `readlink()`) Symlinks are different it seems. Both reading a symlink (with ``readlink()``)
and looking up a symlink on the way to some other destination can and looking up a symlink on the way to some other destination can
update the atime on that symlink. update the atime on that symlink.
[clearest statement]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_08 .. _clearest statement: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_08
It is not clear why this is the case; POSIX has little to say on the It is not clear why this is the case; POSIX has little to say on the
subject. The [clearest statement] is that, if a particular implementation subject. The `clearest statement`_ is that, if a particular implementation
updates a timestamp in a place not specified by POSIX, this must be updates a timestamp in a place not specified by POSIX, this must be
documented "except that any changes caused by pathname resolution need documented "except that any changes caused by pathname resolution need
not be documented". This seems to imply that POSIX doesn't really not be documented". This seems to imply that POSIX doesn't really
care about access-time updates during pathname lookup. care about access-time updates during pathname lookup.
[Linux 1.3.87]: https://git.kernel.org/cgit/linux/kernel/git/history/history.git/diff/fs/ext2/symlink.c?id=f806c6db77b8eaa6e00dcfb6b567706feae8dbb8 .. _Linux 1.3.87: https://git.kernel.org/cgit/linux/kernel/git/history/history.git/diff/fs/ext2/symlink.c?id=f806c6db77b8eaa6e00dcfb6b567706feae8dbb8
An examination of history shows that prior to [Linux 1.3.87], the ext2 An examination of history shows that prior to `Linux 1.3.87`_, the ext2
filesystem, at least, didn't update atime when following a link. filesystem, at least, didn't update atime when following a link.
Unfortunately we have no record of why that behavior was changed. Unfortunately we have no record of why that behavior was changed.
In any case, access time must now be updated and that operation can be In any case, access time must now be updated and that operation can be
quite complex. Trying to stay in RCU-walk while doing it is best quite complex. Trying to stay in RCU-walk while doing it is best
avoided. Fortunately it is often permitted to skip the `atime` avoided. Fortunately it is often permitted to skip the ``atime``
update. Because `atime` updates cause performance problems in various update. Because ``atime`` updates cause performance problems in various
areas, Linux supports the `relatime` mount option, which generally areas, Linux supports the ``relatime`` mount option, which generally
limits the updates of `atime` to once per day on files that aren't limits the updates of ``atime`` to once per day on files that aren't
being changed (and symlinks never change once created). Even without being changed (and symlinks never change once created). Even without
`relatime`, many filesystems record `atime` with a one-second ``relatime``, many filesystems record ``atime`` with a one-second
granularity, so only one update per second is required. granularity, so only one update per second is required.
It is easy to test if an `atime` update is needed while in RCU-walk It is easy to test if an ``atime`` update is needed while in RCU-walk
mode and, if it isn't, the update can be skipped and RCU-walk mode mode and, if it isn't, the update can be skipped and RCU-walk mode
continues. Only when an `atime` update is actually required does the continues. Only when an ``atime`` update is actually required does the
path walk drop down to REF-walk. All of this is handled in the path walk drop down to REF-walk. All of this is handled in the
`get_link()` function. ``get_link()`` function.
A few flags A few flags
----------- -----------
A suitable way to wrap up this tour of pathname walking is to list A suitable way to wrap up this tour of pathname walking is to list
the various flags that can be stored in the `nameidata` to guide the the various flags that can be stored in the ``nameidata`` to guide the
lookup process. Many of these are only meaningful on the final lookup process. Many of these are only meaningful on the final
component, others reflect the current state of the pathname lookup. component, others reflect the current state of the pathname lookup.
And then there is `LOOKUP_EMPTY`, which doesn't fit conceptually with And then there is ``LOOKUP_EMPTY``, which doesn't fit conceptually with
the others. If this is not set, an empty pathname causes an error the others. If this is not set, an empty pathname causes an error
very early on. If it is set, empty pathnames are not considered to be very early on. If it is set, empty pathnames are not considered to be
an error. an error.
### Global state flags ### Global state flags
~~~~~~~~~~~~~~~~~~
We have already met two global state flags: `LOOKUP_RCU` and We have already met two global state flags: ``LOOKUP_RCU`` and
`LOOKUP_REVAL`. These select between one of three overall approaches ``LOOKUP_REVAL``. These select between one of three overall approaches
to lookup: RCU-walk, REF-walk, and REF-walk with forced revalidation. to lookup: RCU-walk, REF-walk, and REF-walk with forced revalidation.
`LOOKUP_PARENT` indicates that the final component hasn't been reached ``LOOKUP_PARENT`` indicates that the final component hasn't been reached
yet. This is primarily used to tell the audit subsystem the full yet. This is primarily used to tell the audit subsystem the full
context of a particular access being audited. context of a particular access being audited.
`LOOKUP_ROOT` indicates that the `root` field in the `nameidata` was ``LOOKUP_ROOT`` indicates that the ``root`` field in the ``nameidata`` was
provided by the caller, so it shouldn't be released when it is no provided by the caller, so it shouldn't be released when it is no
longer needed. longer needed.
`LOOKUP_JUMPED` means that the current dentry was chosen not because ``LOOKUP_JUMPED`` means that the current dentry was chosen not because
it had the right name but for some other reason. This happens when it had the right name but for some other reason. This happens when
following "`..`", following a symlink to `/`, crossing a mount point following "``..``", following a symlink to ``/``, crossing a mount point
or accessing a "`/proc/$PID/fd/$FD`" symlink. In this case the or accessing a "``/proc/$PID/fd/$FD``" symlink. In this case the
filesystem has not been asked to revalidate the name (with filesystem has not been asked to revalidate the name (with
`d_revalidate()`). In such cases the inode may still need to be ``d_revalidate()``). In such cases the inode may still need to be
revalidated, so `d_op->d_weak_revalidate()` is called if revalidated, so ``d_op->d_weak_revalidate()`` is called if
`LOOKUP_JUMPED` is set when the look completes - which may be at the ``LOOKUP_JUMPED`` is set when the look completes - which may be at the
final component or, when creating, unlinking, or renaming, at the penultimate component. final component or, when creating, unlinking, or renaming, at the penultimate component.
### Final-component flags ### Final-component flags
~~~~~~~~~~~~~~~~~~~~~
Some of these flags are only set when the final component is being Some of these flags are only set when the final component is being
considered. Others are only checked for when considering that final considered. Others are only checked for when considering that final
component. component.
`LOOKUP_AUTOMOUNT` ensures that, if the final component is an automount ``LOOKUP_AUTOMOUNT`` ensures that, if the final component is an automount
point, then the mount is triggered. Some operations would trigger it point, then the mount is triggered. Some operations would trigger it
anyway, but operations like `stat()` deliberately don't. `statfs()` anyway, but operations like ``stat()`` deliberately don't. ``statfs()``
needs to trigger the mount but otherwise behaves a lot like `stat()`, so needs to trigger the mount but otherwise behaves a lot like ``stat()``, so
it sets `LOOKUP_AUTOMOUNT`, as does "`quotactl()`" and the handling of it sets ``LOOKUP_AUTOMOUNT``, as does "``quotactl()``" and the handling of
"`mount --bind`". "``mount --bind``".
`LOOKUP_FOLLOW` has a similar function to `LOOKUP_AUTOMOUNT` but for ``LOOKUP_FOLLOW`` has a similar function to ``LOOKUP_AUTOMOUNT`` but for
symlinks. Some system calls set or clear it implicitly, while symlinks. Some system calls set or clear it implicitly, while
others have API flags such as `AT_SYMLINK_FOLLOW` and others have API flags such as ``AT_SYMLINK_FOLLOW`` and
`UMOUNT_NOFOLLOW` to control it. Its effect is similar to ``UMOUNT_NOFOLLOW`` to control it. Its effect is similar to
`WALK_GET` that we already met, but it is used in a different way. ``WALK_GET`` that we already met, but it is used in a different way.
`LOOKUP_DIRECTORY` insists that the final component is a directory. ``LOOKUP_DIRECTORY`` insists that the final component is a directory.
Various callers set this and it is also set when the final component Various callers set this and it is also set when the final component
is found to be followed by a slash. is found to be followed by a slash.
Finally `LOOKUP_OPEN`, `LOOKUP_CREATE`, `LOOKUP_EXCL`, and Finally ``LOOKUP_OPEN``, ``LOOKUP_CREATE``, ``LOOKUP_EXCL``, and
`LOOKUP_RENAME_TARGET` are not used directly by the VFS but are made ``LOOKUP_RENAME_TARGET`` are not used directly by the VFS but are made
available to the filesystem and particularly the `->d_revalidate()` available to the filesystem and particularly the ``->d_revalidate()``
method. A filesystem can choose not to bother revalidating too hard method. A filesystem can choose not to bother revalidating too hard
if it knows that it will be asked to open or create the file soon. if it knows that it will be asked to open or create the file soon.
These flags were previously useful for `->lookup()` too but with the These flags were previously useful for ``->lookup()`` too but with the
introduction of `->atomic_open()` they are less relevant there. introduction of ``->atomic_open()`` they are less relevant there.
End of the road End of the road
--------------- ---------------
......
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