linux-kernel - Re: [PATCH]mm/compation.c: checking page in lru twice

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Date:	Fri, 6 May 2011 20:26:43 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	"Figo.zhang" <figo1802@...il.com>,
	lkml <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	kamezawa.hiroyu@...fujisu.com, minchan.kim@...il.com,
	Andrew Morton <akpm@...l.org>
Subject: Re: [PATCH]mm/compation.c: checking page in lru twice

On Fri, May 06, 2011 at 02:09:55PM +0100, Mel Gorman wrote:
> On Fri, May 06, 2011 at 07:32:46PM +0800, Figo.zhang wrote:
> > 
> > in isolate_migratepages() have check page in LRU twice, the next one
> > at _isolate_lru_page(). 
> > 
> > Signed-off-by: Figo.zhang <figo1802@...il.com> 
> 
> Not checking for PageLRU means that PageTransHuge() gets called
> for each page. While the scanner is active and the lock released,
> a transparent hugepage can be created and potentially we test
> PageTransHuge() on a tail page. This will trigger a BUG if
> CONFIG_DEBUG_VM is set.

Agreed. The compound_order also would become unsafe even if it was
initially an head page (if it's a compound page not in lru). And
compound_trans_order isn't a solution either because we need to be
head for it to be safe like you said, better not having to use
compound_trans_order.
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