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SubjectRe: [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@vflare.org>
>>
>> Changes for v2:
>> - Regroup descriptions as suggested by Seth
> ^^ - Konrad
>
> Otherwise: Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.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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