fscache: Rewrite documentation

Rewrite the fscache documentation.

Changes
=======
ver #3:
 - The volume coherency data is now an arbitrarily-sized blob, not a u64.

ver #2:
 - Put quoting around some bits of C being referred to in the docs[1].
 - Stripped the markup off the ref to the netfs lib doc[2].
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/20211130175119.63d0e7aa@canb.auug.org.au/ [1]
Link: https://lore.kernel.org/r/20211130162311.105fcfa5@canb.auug.org.au/ [2]
Link: https://lore.kernel.org/r/163819672252.215744.15454333549935901588.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906986754.143852.17703291789683936950.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967193834.1823006.15991526817786159772.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021585970.640689.3162537597817521032.stgit@warthog.procyon.org.uk/ # v4

Documentation/filesystems/caching/cachefiles.rst

 .. SPDX-License-Identifier: GPL-2.0
 
-===============================================
-CacheFiles: CACHE ON ALREADY MOUNTED FILESYSTEM
-===============================================
+===================================
+Cache on Already Mounted Filesystem
+===================================
 
 .. Contents:
 

Documentation/filesystems/caching/index.rst

@@ -7,8 +7,6 @@ Filesystem Caching
    :maxdepth: 2
 
    fscache
-   object
+   netfs-api
    backend-api
    cachefiles
-   netfs-api
-   operations

Documentation/filesystems/caching/operations.rst

-.. SPDX-License-Identifier: GPL-2.0
-
-================================
-Asynchronous Operations Handling
-================================
-
-By: David Howells <dhowells@redhat.com>
-
-.. Contents:
-
- (*) Overview.
-
- (*) Operation record initialisation.
-
- (*) Parameters.
-
- (*) Procedure.
-
- (*) Asynchronous callback.
-
-
-Overview
-========
-
-FS-Cache has an asynchronous operations handling facility that it uses for its
-data storage and retrieval routines.  Its operations are represented by
-fscache_operation structs, though these are usually embedded into some other
-structure.
-
-This facility is available to and expected to be used by the cache backends,
-and FS-Cache will create operations and pass them off to the appropriate cache
-backend for completion.
-
-To make use of this facility, <linux/fscache-cache.h> should be #included.
-
-
-Operation Record Initialisation
-===============================
-
-An operation is recorded in an fscache_operation struct::
-
-	struct fscache_operation {
-		union {
-			struct work_struct fast_work;
-			struct slow_work slow_work;
-		};
-		unsigned long		flags;
-		fscache_operation_processor_t processor;
-		...
-	};
-
-Someone wanting to issue an operation should allocate something with this
-struct embedded in it.  They should initialise it by calling::
-
-	void fscache_operation_init(struct fscache_operation *op,
-				    fscache_operation_release_t release);
-
-with the operation to be initialised and the release function to use.
-
-The op->flags parameter should be set to indicate the CPU time provision and
-the exclusivity (see the Parameters section).
-
-The op->fast_work, op->slow_work and op->processor flags should be set as
-appropriate for the CPU time provision (see the Parameters section).
-
-FSCACHE_OP_WAITING may be set in op->flags prior to each submission of the
-operation and waited for afterwards.
-
-
-Parameters
-==========
-
-There are a number of parameters that can be set in the operation record's flag
-parameter.  There are three options for the provision of CPU time in these
-operations:
-
- (1) The operation may be done synchronously (FSCACHE_OP_MYTHREAD).  A thread
-     may decide it wants to handle an operation itself without deferring it to
-     another thread.
-
-     This is, for example, used in read operations for calling readpages() on
-     the backing filesystem in CacheFiles.  Although readpages() does an
-     asynchronous data fetch, the determination of whether pages exist is done
-     synchronously - and the netfs does not proceed until this has been
-     determined.
-
-     If this option is to be used, FSCACHE_OP_WAITING must be set in op->flags
-     before submitting the operation, and the operating thread must wait for it
-     to be cleared before proceeding::
-
-		wait_on_bit(&op->flags, FSCACHE_OP_WAITING,
-			    TASK_UNINTERRUPTIBLE);
-
-
- (2) The operation may be fast asynchronous (FSCACHE_OP_FAST), in which case it
-     will be given to keventd to process.  Such an operation is not permitted
-     to sleep on I/O.
-
-     This is, for example, used by CacheFiles to copy data from a backing fs
-     page to a netfs page after the backing fs has read the page in.
-
-     If this option is used, op->fast_work and op->processor must be
-     initialised before submitting the operation::
-
-		INIT_WORK(&op->fast_work, do_some_work);
-
-
- (3) The operation may be slow asynchronous (FSCACHE_OP_SLOW), in which case it
-     will be given to the slow work facility to process.  Such an operation is
-     permitted to sleep on I/O.
-
-     This is, for example, used by FS-Cache to handle background writes of
-     pages that have just been fetched from a remote server.
-
-     If this option is used, op->slow_work and op->processor must be
-     initialised before submitting the operation::
-
-		fscache_operation_init_slow(op, processor)
-
-
-Furthermore, operations may be one of two types:
-
- (1) Exclusive (FSCACHE_OP_EXCLUSIVE).  Operations of this type may not run in
-     conjunction with any other operation on the object being operated upon.
-
-     An example of this is the attribute change operation, in which the file
-     being written to may need truncation.
-
- (2) Shareable.  Operations of this type may be running simultaneously.  It's
-     up to the operation implementation to prevent interference between other
-     operations running at the same time.
-
-
-Procedure
-=========
-
-Operations are used through the following procedure:
-
- (1) The submitting thread must allocate the operation and initialise it
-     itself.  Normally this would be part of a more specific structure with the
-     generic op embedded within.
-
- (2) The submitting thread must then submit the operation for processing using
-     one of the following two functions::
-
-	int fscache_submit_op(struct fscache_object *object,
-			      struct fscache_operation *op);
-
-	int fscache_submit_exclusive_op(struct fscache_object *object,
-					struct fscache_operation *op);
-
-     The first function should be used to submit non-exclusive ops and the
-     second to submit exclusive ones.  The caller must still set the
-     FSCACHE_OP_EXCLUSIVE flag.
-
-     If successful, both functions will assign the operation to the specified
-     object and return 0.  -ENOBUFS will be returned if the object specified is
-     permanently unavailable.
-
-     The operation manager will defer operations on an object that is still
-     undergoing lookup or creation.  The operation will also be deferred if an
-     operation of conflicting exclusivity is in progress on the object.
-
-     If the operation is asynchronous, the manager will retain a reference to
-     it, so the caller should put their reference to it by passing it to::
-
-	void fscache_put_operation(struct fscache_operation *op);
-
- (3) If the submitting thread wants to do the work itself, and has marked the
-     operation with FSCACHE_OP_MYTHREAD, then it should monitor
-     FSCACHE_OP_WAITING as described above and check the state of the object if
-     necessary (the object might have died while the thread was waiting).
-
-     When it has finished doing its processing, it should call
-     fscache_op_complete() and fscache_put_operation() on it.
-
- (4) The operation holds an effective lock upon the object, preventing other
-     exclusive ops conflicting until it is released.  The operation can be
-     enqueued for further immediate asynchronous processing by adjusting the
-     CPU time provisioning option if necessary, eg::
-
-	op->flags &= ~FSCACHE_OP_TYPE;
-	op->flags |= ~FSCACHE_OP_FAST;
-
-     and calling::
-
-	void fscache_enqueue_operation(struct fscache_operation *op)
-
-     This can be used to allow other things to have use of the worker thread
-     pools.
-
-
-Asynchronous Callback
-=====================
-
-When used in asynchronous mode, the worker thread pool will invoke the
-processor method with a pointer to the operation.  This should then get at the
-container struct by using container_of()::
-
-	static void fscache_write_op(struct fscache_operation *_op)
-	{
-		struct fscache_storage *op =
-			container_of(_op, struct fscache_storage, op);
-	...
-	}
-
-The caller holds a reference on the operation, and will invoke
-fscache_put_operation() when the processor function returns.  The processor
-function is at liberty to call fscache_enqueue_operation() or to take extra
-references.

Documentation/filesystems/netfs_library.rst

@@ -454,7 +454,8 @@ operation table looks like the following::
 			    void *term_func_priv);
 
 		int (*prepare_write)(struct netfs_cache_resources *cres,
-				     loff_t *_start, size_t *_len, loff_t i_size);
+				     loff_t *_start, size_t *_len, loff_t i_size,
+				     bool no_space_allocated_yet);
 
 		int (*write)(struct netfs_cache_resources *cres,
 			     loff_t start_pos,
@@ -515,11 +516,14 @@ The methods defined in the table are:
 
  * ``prepare_write()``
 
-   [Required] Called to adjust a write to the cache and check that there is
-   sufficient space in the cache.  The start and length values indicate the
-   size of the write that netfslib is proposing, and this can be adjusted by
-   the cache to respect DIO boundaries.  The file size is passed for
-   information.
+   [Required] Called to prepare a write to the cache to take place.  This
+   involves checking to see whether the cache has sufficient space to honour
+   the write.  ``*_start`` and ``*_len`` indicate the region to be written; the
+   region can be shrunk or it can be expanded to a page boundary either way as
+   necessary to align for direct I/O.  i_size holds the size of the object and
+   is provided for reference.  no_space_allocated_yet is set to true if the
+   caller is certain that no data has been written to that region - for example
+   if it tried to do a read from there already.
 
  * ``write()``