linux-kernel - Re: [PATCH] mm: skip the page buddy block instead of one page

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Date:	Wed, 14 Aug 2013 23:22:41 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Minchan Kim <minchan@...nel.org>, Xishi Qiu <qiuxishi@...wei.com>,
	riel@...hat.com, aquini@...hat.com, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: skip the page buddy block instead of one page

On Wed, Aug 14, 2013 at 01:26:02PM -0700, Andrew Morton wrote:
> On Thu, 15 Aug 2013 00:52:29 +0900 Minchan Kim <minchan@...nel.org> wrote:
> 
> > On Wed, Aug 14, 2013 at 09:57:11AM +0100, Mel Gorman wrote:
> > > On Wed, Aug 14, 2013 at 12:45:41PM +0800, Xishi Qiu wrote:
> > > > A large free page buddy block will continue many times, so if the page 
> > > > is free, skip the whole page buddy block instead of one page.
> > > > 
> > > > Signed-off-by: Xishi Qiu <qiuxishi@...wei.com>
> > > 
> > > page_order cannot be used unless zone->lock is held which is not held in
> > > this path. Acquiring the lock would prevent parallel allocations from the
> > 
> > Argh, I missed that.
> 
> I missed it as well. And so did Xishi Qiu.
> 
> Mel, we have a problem.  What can we do to make this code more
> maintainable?

I sit in the bad man corner until I write a comment patch :/

page_order already has a comment but obviously the call site on compaction.c
could do with a hint. As I think the consequences of this race can be
dealt with I'm hoping Xishi Qiu will take the example I posted, fix it
if it needs fixing, turn it into a real patch and run it through whatever
test case led him to find this problem in the first place (HINT HINT). If
that happens, great!  If not, I might do it myself and failing that, I'll
post a patch adding a comment explaining why page_order is not used there.

-- 
Mel Gorman
SUSE Labs
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