Re: [PATCH v2] zsmalloc documentation

[Konrad Rzeszutek Wilk]

On Sat, Jun 09, 2012 at 05:41:14PM -0700, Nitin Gupta wrote:
> Documentation of various struct page fields
> used by zsmalloc.
> 
> Signed-off-by: Nitin Gupta <ngupta@xxxxxxxxxx>
> 
> Changes for v2:
> 	- Regroup descriptions as suggested by Seth
                                               ^^ - Konrad

Otherwise: Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>

> ---
>  drivers/staging/zsmalloc/zsmalloc-main.c |   48 ++++++++++++++++++++++++++++++
>  1 file changed, 48 insertions(+)
> 
> diff --git a/drivers/staging/zsmalloc/zsmalloc-main.c b/drivers/staging/zsmalloc/zsmalloc-main.c
> index 4496737..1db76ec 100644
> --- a/drivers/staging/zsmalloc/zsmalloc-main.c
> +++ b/drivers/staging/zsmalloc/zsmalloc-main.c
> @@ -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
> -- 
> 1.7.10.2
> 
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