Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
1 zsmalloc 2 -------- 3 4 This allocator is designed for use with zram. Thus, the allocator is 5 supposed to work well under low memory conditions. In particular, it 6 never attempts higher order page allocation which is very likely to 7 fail under memory pressure. On the other hand, if we just use single 8 (0-order) pages, it would suffer from very high fragmentation -- 9 any object of size PAGE_SIZE/2 or larger would occupy an entire page. 10 This was one of the major issues with its predecessor (xvmalloc). 11 12 To overcome these issues, zsmalloc allocates a bunch of 0-order pages 13 and links them together using various 'struct page' fields. These linked 14 pages act as a single higher-order page i.e. an object can span 0-order 15 page boundaries. The code refers to these linked pages as a single entity 16 called zspage. 17 18 For simplicity, zsmalloc can only allocate objects of size up to PAGE_SIZE 19 since this satisfies the requirements of all its current users (in the 20 worst case, page is incompressible and is thus stored "as-is" i.e. in 21 uncompressed form). For allocation requests larger than this size, failure 22 is returned (see zs_malloc). 23 24 Additionally, zs_malloc() does not return a dereferenceable pointer. 25 Instead, it returns an opaque handle (unsigned long) which encodes actual 26 location of the allocated object. The reason for this indirection is that 27 zsmalloc does not keep zspages permanently mapped since that would cause 28 issues on 32-bit systems where the VA region for kernel space mappings 29 is very small. So, before using the allocating memory, the object has to 30 be mapped using zs_map_object() to get a usable pointer and subsequently 31 unmapped using zs_unmap_object(). 32 33 stat 34 ---- 35 36 With CONFIG_ZSMALLOC_STAT, we could see zsmalloc internal information via 37 /sys/kernel/debug/zsmalloc/<user name>. Here is a sample of stat output: 38 39 # cat /sys/kernel/debug/zsmalloc/zram0/classes 40 41 class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage 42 .. 43 .. 44 9 176 0 1 186 129 8 4 45 10 192 1 0 2880 2872 135 3 46 11 208 0 1 819 795 42 2 47 12 224 0 1 219 159 12 4 48 .. 49 .. 50 51 52 class: index 53 size: object size zspage stores 54 almost_empty: the number of ZS_ALMOST_EMPTY zspages(see below) 55 almost_full: the number of ZS_ALMOST_FULL zspages(see below) 56 obj_allocated: the number of objects allocated 57 obj_used: the number of objects allocated to the user 58 pages_used: the number of pages allocated for the class 59 pages_per_zspage: the number of 0-order pages to make a zspage 60 61 We assign a zspage to ZS_ALMOST_EMPTY fullness group when: 62 n <= N / f, where 63 n = number of allocated objects 64 N = total number of objects zspage can store 65 f = fullness_threshold_frac(ie, 4 at the moment) 66 67 Similarly, we assign zspage to: 68 ZS_ALMOST_FULL when n > N / f 69 ZS_EMPTY when n == 0 70 ZS_FULL when n == N