diff options
| -rw-r--r-- | Documentation/vm/zsmalloc.txt | 70 | ||||
| -rw-r--r-- | MAINTAINERS | 1 | ||||
| -rw-r--r-- | mm/zsmalloc.c | 29 |
3 files changed, 71 insertions, 29 deletions
diff --git a/Documentation/vm/zsmalloc.txt b/Documentation/vm/zsmalloc.txt new file mode 100644 index 000000000000..64ed63c4f69d --- /dev/null +++ b/Documentation/vm/zsmalloc.txt @@ -0,0 +1,70 @@ +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 diff --git a/MAINTAINERS b/MAINTAINERS index 6ee1e79ea16b..190981382853 100644 --- a/MAINTAINERS +++ b/MAINTAINERS @@ -10972,6 +10972,7 @@ L: linux-mm@kvack.org S: Maintained F: mm/zsmalloc.c F: include/linux/zsmalloc.h +F: Documentation/vm/zsmalloc.txt ZSWAP COMPRESSED SWAP CACHING M: Seth Jennings <sjennings@variantweb.net> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c index 461243e14d3e..1833fc9e09cb 100644 --- a/mm/zsmalloc.c +++ b/mm/zsmalloc.c @@ -12,35 +12,6 @@ */ /* - * 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(). - * * Following is how we use various fields and flags of underlying * struct page(s) to form a zspage. * |