zsmalloc¶

This allocator is designed for use with zram. Thus, the allocator issupposed to work well under low memory conditions. In particular, itnever attempts higher order page allocation which is very likely tofail 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 pagesand links them together using various ‘struct page’ fields. These linkedpages act as a single higher-order page i.e. an object can span 0-orderpage boundaries. The code refers to these linked pages as a single entitycalled zspage.

For simplicity, zsmalloc can only allocate objects of size up to PAGE_SIZEsince this satisfies the requirements of all its current users (in theworst case, page is incompressible and is thus stored “as-is” i.e. inuncompressed form). For allocation requests larger than this size, failureis returned (see zs_malloc).

Additionally, zs_malloc() does not return a dereferenceable pointer.Instead, it returns an opaque handle (unsigned long) which encodes actuallocation of the allocated object. The reason for this indirection is thatzsmalloc does not keep zspages permanently mapped since that would causeissues on 32-bit systems where the VA region for kernel space mappingsis very small. So, before using the allocating memory, the object has tobe mapped using zs_map_object() to get a usable pointer and subsequentlyunmapped using zs_unmap_object().

# cat /sys/kernel/debug/zsmalloc/zram0/classesclass  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   ...   ...