1.. _zsmalloc: 2 3======== 4zsmalloc 5======== 6 7This allocator is designed for use with zram. Thus, the allocator is 8supposed to work well under low memory conditions. In particular, it 9never attempts higher order page allocation which is very likely to 10fail under memory pressure. On the other hand, if we just use single 11(0-order) pages, it would suffer from very high fragmentation -- 12any object of size PAGE_SIZE/2 or larger would occupy an entire page. 13This was one of the major issues with its predecessor (xvmalloc). 14 15To overcome these issues, zsmalloc allocates a bunch of 0-order pages 16and links them together using various 'struct page' fields. These linked 17pages act as a single higher-order page i.e. an object can span 0-order 18page boundaries. The code refers to these linked pages as a single entity 19called zspage. 20 21For simplicity, zsmalloc can only allocate objects of size up to PAGE_SIZE 22since this satisfies the requirements of all its current users (in the 23worst case, page is incompressible and is thus stored "as-is" i.e. in 24uncompressed form). For allocation requests larger than this size, failure 25is returned (see zs_malloc). 26 27Additionally, zs_malloc() does not return a dereferenceable pointer. 28Instead, it returns an opaque handle (unsigned long) which encodes actual 29location of the allocated object. The reason for this indirection is that 30zsmalloc does not keep zspages permanently mapped since that would cause 31issues on 32-bit systems where the VA region for kernel space mappings 32is very small. So, before using the allocating memory, the object has to 33be mapped using zs_map_object() to get a usable pointer and subsequently 34unmapped using zs_unmap_object(). 35 36stat 37==== 38 39With 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 55class 56 index 57size 58 object size zspage stores 59almost_empty 60 the number of ZS_ALMOST_EMPTY zspages(see below) 61almost_full 62 the number of ZS_ALMOST_FULL zspages(see below) 63obj_allocated 64 the number of objects allocated 65obj_used 66 the number of objects allocated to the user 67pages_used 68 the number of pages allocated for the class 69pages_per_zspage 70 the number of 0-order pages to make a zspage 71 72We 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 78Similarly, 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 83