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Commit 2db51dae authored by Nitin Gupta's avatar Nitin Gupta Committed by Greg Kroah-Hartman
Browse files

staging: zsmalloc documentation 



Documentation of various struct page fields
used by zsmalloc.

Changes for v2:
	- Regroup descriptions as suggested by Konrad

Signed-off-by: default avatarNitin Gupta <ngupta@vflare.org>
Acked-by: default avatarKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: default avatarMinchan Kim <minchan@kernel.org>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 130f315a

drivers/staging/zsmalloc/zsmalloc-main.c

+48 −0
Original line number Diff line number Diff line
@@ -10,6 +10,54 @@ 
 * 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

 *	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.

 *	page->lru: links together all component pages (except the first page)

 *		of a zspage

 *

 *	For _first_ page only:

 *

 *	page->private (union with page->first_page): refers to the

 *		component page after the first page

 *	page->freelist: points to the first free object in zspage.

 *		Free objects are linked together using in-place

 *		metadata.

 *	page->objects: maximum number of objects we can store in this

 *		zspage (class->zspage_order * PAGE_SIZE / class->size)

 *	page->lru: links together first pages of various zspages.

 *		Basically forming list of zspages in a fullness group.

 *	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