Signed-off-by: Nitin Gupta <ngupta@xxxxxxxxxx> --- drivers/staging/zsmalloc/zsmalloc-main.c | 49 ++++++++++++++++++++++++++++++ 1 file changed, 49 insertions(+) diff --git a/drivers/staging/zsmalloc/zsmalloc-main.c b/drivers/staging/zsmalloc/zsmalloc-main.c index 4496737..f80f2fd 100644 --- a/drivers/staging/zsmalloc/zsmalloc-main.c +++ b/drivers/staging/zsmalloc/zsmalloc-main.c @@ -10,6 +10,55 @@ * Released under the terms of GNU General Public License Version 2.0 */ + +/* + * This allocator is designed for use with zcache and 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. + * + * Following is how we use various fields and flags of underlying + * struct page(s) to form a zspage. + * + * Usage of struct page fields: + * page->first_page: points to the first component (0-order) page + * [first] page->private (union with page->first_page): refers to the + * component page after the first page + * + * [first] page->freelist: points to the first free object in zspage. + * Free objects are linked together using in-place + * metadata. + * page->index (union with page->freelist): offset of the first object + * starting in this page. For the first page, this is + * always 0, so we use this field (aka freelist) to point + * to the first free object in zspage. + * + * [first] page->objects: maximum number of objects we can store in this + * zspage (class->zspage_order * PAGE_SIZE / class->size) + * + * page->lru: links together all component pages (except the first page) + * of a zspage + * [first] page->lru: links together first pages of various zspages. + * Basically forming list of zspages in a fullness group. + * + * [first] page->mapping: class index and fullness group of the zspage + * + * Usage of struct page flags: + * PG_private: identifies the first component page + * PG_private2: identifies the last component page + * + */ + #ifdef CONFIG_ZSMALLOC_DEBUG #define DEBUG #endif -- 1.7.10.2 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>