linux-kernel - Re: [PATCH v2] zsmalloc documentation

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Date:	Sat, 09 Jun 2012 17:49:42 -0700
From:	Nitin Gupta <ngupta@...are.org>
To:	Konrad Rzeszutek Wilk <konrad@...nok.org>
CC:	Greg KH <greg@...ah.com>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Minchan Kim <minchan.kim@...il.com>,
	linux-mm <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] zsmalloc documentation

On 06/09/2012 05:44 PM, Konrad Rzeszutek Wilk wrote:

> 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@...are.org>
>>
>> Changes for v2:
>> 	- Regroup descriptions as suggested by Seth
>                                                ^^ - Konrad
> 
> Otherwise: Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> 


Sorry about that, Konrad!
Greg: Please let me know if I should resend the patch with corrected
name in the changelog.

Thanks,
Nitin

>> ---
>>  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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