zsmalloc: zsmalloc documentation

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

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>

 */

/*

 * 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.

 *