mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-11-01 00:48:50 +00:00
ad56b738c5
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
82 lines
3 KiB
ReStructuredText
82 lines
3 KiB
ReStructuredText
.. _zsmalloc:
|
|
|
|
========
|
|
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
|