Documentation/mm/zsmalloc.rst

   1 .. _zsmalloc:
   2
   3 ========
   4 zsmalloc
   5 ========
   6
   7 This allocator is designed for use with zram. Thus, the allocator is
   8 supposed to work well under low memory conditions. In particular, it
   9 never attempts higher order page allocation which is very likely to
  10 fail under memory pressure. On the other hand, if we just use single
  11 (0-order) pages, it would suffer from very high fragmentation --
  12 any object of size PAGE_SIZE/2 or larger would occupy an entire page.
  13 This was one of the major issues with its predecessor (xvmalloc).
  14
  15 To overcome these issues, zsmalloc allocates a bunch of 0-order pages
  16 and links them together using various 'struct page' fields. These linked
  17 pages act as a single higher-order page i.e. an object can span 0-order
  18 page boundaries. The code refers to these linked pages as a single entity
  19 called zspage.
  20
  21 For simplicity, zsmalloc can only allocate objects of size up to PAGE_SIZE
  22 since this satisfies the requirements of all its current users (in the
  23 worst case, page is incompressible and is thus stored "as-is" i.e. in
  24 uncompressed form). For allocation requests larger than this size, failure
  25 is returned (see zs_malloc).
  26
  27 Additionally, zs_malloc() does not return a dereferenceable pointer.
  28 Instead, it returns an opaque handle (unsigned long) which encodes actual
  29 location of the allocated object. The reason for this indirection is that
  30 zsmalloc does not keep zspages permanently mapped since that would cause
  31 issues on 32-bit systems where the VA region for kernel space mappings
  32 is very small. So, before using the allocating memory, the object has to
  33 be mapped using zs_map_object() to get a usable pointer and subsequently
  34 unmapped using zs_unmap_object().
  35
  36 stat
  37 ====
  38
  39 With CONFIG_ZSMALLOC_STAT, we could see zsmalloc internal information via
  40 ``/sys/kernel/debug/zsmalloc/<user name>``. Here is a sample of stat output::
  41
  42  # cat /sys/kernel/debug/zsmalloc/zram0/classes
  43
  44  class  size almost_full almost_empty obj_allocated   obj_used pages_used pages_per_zspage
  45     ...
  46     ...
  47      9   176           0            1           186        129          8                4
  48     10   192           1            0          2880       2872        135                3
  49     11   208           0            1           819        795         42                2
  50     12   224           0            1           219        159         12                4
  51     ...
  52     ...
  53
  54
  55 class
  56         index
  57 size
  58         object size zspage stores
  59 almost_empty
  60         the number of ZS_ALMOST_EMPTY zspages(see below)
  61 almost_full
  62         the number of ZS_ALMOST_FULL zspages(see below)
  63 obj_allocated
  64         the number of objects allocated
  65 obj_used
  66         the number of objects allocated to the user
  67 pages_used
  68         the number of pages allocated for the class
  69 pages_per_zspage
  70         the number of 0-order pages to make a zspage
  71
  72 We assign a zspage to ZS_ALMOST_EMPTY fullness group when n <= N / f, where
  73
  74 * n = number of allocated objects
  75 * N = total number of objects zspage can store
  76 * f = fullness_threshold_frac(ie, 4 at the moment)
  77
  78 Similarly, we assign zspage to:
  79
  80 * ZS_ALMOST_FULL  when n > N / f
  81 * ZS_EMPTY        when n == 0
  82 * ZS_FULL         when n == N