Commit d02be50d authored by Minchan Kim's avatar Minchan Kim Committed by Linus Torvalds

zsmalloc: zsmalloc documentation

Create zsmalloc doc which explains design concept and stat information.
Signed-off-by: default avatarMinchan Kim <minchan@kernel.org>
Cc: Juneho Choi <juno.choi@lge.com>
Cc: Gunho Lee <gunho.lee@lge.com>
Cc: Luigi Semenzato <semenzato@google.com>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Seth Jennings <sjennings@variantweb.net>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Jerome Marchand <jmarchan@redhat.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 248ca1b0
zsmalloc
--------
This allocator is designed for use with zram. Thus, the allocator is
supposed to work well under low memory conditions. In particular, it
never attempts higher order page allocation which is very likely to
fail under memory pressure. On the other hand, if we just use single
(0-order) pages, it would suffer from very high fragmentation --
any object of size PAGE_SIZE/2 or larger would occupy an entire page.
This was one of the major issues with its predecessor (xvmalloc).
To overcome these issues, zsmalloc allocates a bunch of 0-order pages
and links them together using various 'struct page' fields. These linked
pages act as a single higher-order page i.e. an object can span 0-order
page boundaries. The code refers to these linked pages as a single entity
called zspage.
For simplicity, zsmalloc can only allocate objects of size up to PAGE_SIZE
since this satisfies the requirements of all its current users (in the
worst case, page is incompressible and is thus stored "as-is" i.e. in
uncompressed form). For allocation requests larger than this size, failure
is returned (see zs_malloc).
Additionally, zs_malloc() does not return a dereferenceable pointer.
Instead, it returns an opaque handle (unsigned long) which encodes actual
location of the allocated object. The reason for this indirection is that
zsmalloc does not keep zspages permanently mapped since that would cause
issues on 32-bit systems where the VA region for kernel space mappings
is very small. So, before using the allocating memory, the object has to
be mapped using zs_map_object() to get a usable pointer and subsequently
unmapped using zs_unmap_object().
stat
----
With CONFIG_ZSMALLOC_STAT, we could see zsmalloc internal information via
/sys/kernel/debug/zsmalloc/<user name>. Here is a sample of stat output:
# cat /sys/kernel/debug/zsmalloc/zram0/classes
class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage
..
..
9 176 0 1 186 129 8 4
10 192 1 0 2880 2872 135 3
11 208 0 1 819 795 42 2
12 224 0 1 219 159 12 4
..
..
class: index
size: object size zspage stores
almost_empty: the number of ZS_ALMOST_EMPTY zspages(see below)
almost_full: the number of ZS_ALMOST_FULL zspages(see below)
obj_allocated: the number of objects allocated
obj_used: the number of objects allocated to the user
pages_used: the number of pages allocated for the class
pages_per_zspage: the number of 0-order pages to make a zspage
We assign a zspage to ZS_ALMOST_EMPTY fullness group when:
n <= N / f, where
n = number of allocated objects
N = total number of objects zspage can store
f = fullness_threshold_frac(ie, 4 at the moment)
Similarly, we assign zspage to:
ZS_ALMOST_FULL when n > N / f
ZS_EMPTY when n == 0
ZS_FULL when n == N
......@@ -10972,6 +10972,7 @@ L: linux-mm@kvack.org
S: Maintained
F: mm/zsmalloc.c
F: include/linux/zsmalloc.h
F: Documentation/vm/zsmalloc.txt
ZSWAP COMPRESSED SWAP CACHING
M: Seth Jennings <sjennings@variantweb.net>
......
......@@ -12,35 +12,6 @@
*/
/*
* This allocator is designed for use with zram. Thus, the allocator is
* supposed to work well under low memory conditions. In particular, it
* never attempts higher order page allocation which is very likely to
* fail under memory pressure. On the other hand, if we just use single
* (0-order) pages, it would suffer from very high fragmentation --
* any object of size PAGE_SIZE/2 or larger would occupy an entire page.
* This was one of the major issues with its predecessor (xvmalloc).
*
* To overcome these issues, zsmalloc allocates a bunch of 0-order pages
* and links them together using various 'struct page' fields. These linked
* pages act as a single higher-order page i.e. an object can span 0-order
* page boundaries. The code refers to these linked pages as a single entity
* called zspage.
*
* For simplicity, zsmalloc can only allocate objects of size up to PAGE_SIZE
* since this satisfies the requirements of all its current users (in the
* worst case, page is incompressible and is thus stored "as-is" i.e. in
* uncompressed form). For allocation requests larger than this size, failure
* is returned (see zs_malloc).
*
* Additionally, zs_malloc() does not return a dereferenceable pointer.
* Instead, it returns an opaque handle (unsigned long) which encodes actual
* location of the allocated object. The reason for this indirection is that
* zsmalloc does not keep zspages permanently mapped since that would cause
* issues on 32-bit systems where the VA region for kernel space mappings
* is very small. So, before using the allocating memory, the object has to
* be mapped using zs_map_object() to get a usable pointer and subsequently
* unmapped using zs_unmap_object().
*
* Following is how we use various fields and flags of underlying
* struct page(s) to form a zspage.
*
......
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