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

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Message-ID: <Pine.LNX.4.64.0611241956420.3938@skynet.skynet.ie>
Date:	Fri, 24 Nov 2006 20:04:49 +0000 (GMT)
From:	Mel Gorman <mel@....ul.ie>
To:	Christoph Lameter <clameter@....com>
Cc:	Linus Torvalds <torvalds@...l.org>, Mel Gorman <mel@...net.ie>,
	linux-mm@...ck.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/11] Add __GFP_MOVABLE flag and update callers

On Fri, 24 Nov 2006, Christoph Lameter wrote:

> On Thu, 23 Nov 2006, Linus Torvalds wrote:
>
>> And the assumption would be that if it's MOVABLE, then it's obviously a
>> USER allocation (it it can fail much more eagerly - that's really what the
>> whole USER bit ends up meaning internally).
>
> We can probably make several types of kernel allocations movable if there
> would be some benefit from it.
>

Page tables are the major type of allocation that comes to mind. From what 
I've seen, they are the most common long-lived unmovable and unreclaimable 
allocation.

> Mel already has a problem with mlocked user pages in the movable section.
> If this is fixed by using page migration to move the mlocked pages

That is the long-term plan.

> then we
> can likely make addititional classes kernel pages also movable and reduce
> the amount of memory that is unmovable. If we have more movable pages then
> the defrag can work more efficiently.

Indeed, although some sort of placement is still needed to keep these 
movable allocations together.

> Having most pages movable will also
> help to make memory unplug a reality.
>
> So please do not require movable pages to be user allocations.
>

That is not the intention. It just happens that allocations that are 
directly accessible by userspace are also the ones that are currently 
movable.

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