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staging: zsmalloc documentation

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Documentation of various struct page fields
used by zsmalloc.

Changes for v2:
	- Regroup descriptions as suggested by Konrad

Signed-off-by: Nitin Gupta <ngupta@vflare.org>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Nitin Gupta 2012-06-09 17:41:14 -07:00 committed by Greg Kroah-Hartman
parent 130f315a17
commit 2db51dae56
1 changed files with 48 additions and 0 deletions
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48
drivers/staging/zsmalloc/zsmalloc-main.c
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@ -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