summaryrefslogtreecommitdiff
path: root/Documentation/mm/zsmalloc.rst
diff options
context:
space:
mode:
Diffstat (limited to 'Documentation/mm/zsmalloc.rst')
-rw-r--r--Documentation/mm/zsmalloc.rst82
1 files changed, 82 insertions, 0 deletions
diff --git a/Documentation/mm/zsmalloc.rst b/Documentation/mm/zsmalloc.rst
new file mode 100644
index 000000000000..6e79893d6132
--- /dev/null
+++ b/Documentation/mm/zsmalloc.rst
@@ -0,0 +1,82 @@
+.. _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