Re: [PATCH 1/11] Add __GFP_MOVABLE flag and update callers

[Mel Gorman <mel@csn.ul.ie>]

On Fri, 24 Nov 2006, Hugh Dickins wrote:

> On Fri, 24 Nov 2006, Mel Gorman wrote:
>>
>> Good catch. In the page clustering patches I work on, I am doing this;
>>
>> -       page = alloc_page_vma(gfp | __GFP_ZERO, &pvma, 0);
>> +       page = alloc_page_vma(
>> +                       set_migrateflags(gfp | __GFP_ZERO, __GFP_RECLAIMABLE),
>> +                                                               &pvma, 0);
>>
>> to get rid of the MOVABLE flag and replace it with __GFP_RECLAIMABLE. This
>> clustered the allocations together with allocations like inode cache. In
>> retrospect, this was not a good idea because it assumes that tmpfs and shmem
>> pages are short-lived. That may not be the case at all.
>> ...
>> Thanks for that clarification. I suspected that something like this was the
>> case when I removed the MOVABLE flag and used RECLAIMABLE but I wasn't 100%
>> certain. In the tests I was running, tmpfs pages weren't a major problem so I
>> didn't chase it down.
>
> I'm fairly confused as to what MOVABLE versus RECLAIMABLE is supposed to
> be meaning, and understand it's in flux, so haven't tried too hard.

A MOVABLE allocation may be moved with page migration or paged out by 
kswapd.

RECLAIMABLE on the other hand applies to short-lived allocations (like a 
socket buffer) or allocations for slab caches that may be reaped such as 
inode caches or dcache.

> Just
> so long as you understand that tmpfs data pages go out to swap under memory
> pressure, whereas ramfs pages do not, and tmpfs swap vector pages do not.
>

Right, I'll take a much closer look with this in mind and make the 
distinction. Thanks

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

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