linux-kernel - Re: [PATCH 1/9] ksm: fix mlockfreed to munlocked

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Date:	Fri, 27 Nov 2009 12:45:04 +0000 (GMT)
From:	Hugh Dickins <hugh.dickins@...cali.co.uk>
To:	Mel Gorman <mel@....ul.ie>, Rik van Riel <riel@...hat.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Izik Eidus <ieidus@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Chris Wright <chrisw@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 1/9] ksm: fix mlockfreed to munlocked

On Thu, 26 Nov 2009, Mel Gorman wrote:
> On Tue, Nov 24, 2009 at 04:40:55PM +0000, Hugh Dickins wrote:
> > When KSM merges an mlocked page, it has been forgetting to munlock it:
> > that's been left to free_page_mlock(), which reports it in /proc/vmstat
> > as unevictable_pgs_mlockfreed instead of unevictable_pgs_munlocked (and
> > whinges "Page flag mlocked set for process" in mmotm, whereas mainline
> > is silently forgiving).  Call munlock_vma_page() to fix that.
> > 
> > Signed-off-by: Hugh Dickins <hugh.dickins@...cali.co.uk>
> 
> Acked-by: Mel Gorman <mel@....ul.ie>

Rik & Mel, thanks for the Acks.

But please clarify: that patch was for mmotm and hopefully 2.6.33,
but the vmstat issue (minus warning message) is there in 2.6.32-rc.
Should I

(a) forget it for 2.6.32
(b) rush Linus a patch for 2.6.32 final
(c) send a patch for 2.6.32.stable later on

? I just don't have a feel for how important this is.

Typically, these pages are immediately freed, and the only issue is
which stats they get added to; but if fork has copied them into other
mms, then such pages might stay unevictable indefinitely, despite no
longer being in any mlocked vma.

There's a remark in munlock_vma_page(), apropos a different issue,
			/*
			 * We lost the race.  let try_to_unmap() deal
			 * with it.  At least we get the page state and
			 * mlock stats right.  However, page is still on
			 * the noreclaim list.  We'll fix that up when
			 * the page is eventually freed or we scan the
			 * noreclaim list.
			 */
which implies that sometimes we scan the unevictable list and resolve
such cases.  But I wonder if that's nowadays the case?

> 
> > ---
> > Is this a fix that I ought to backport to 2.6.32?  It does rely on part of
> > an earlier patch (moved unlock_page down), so does not apply cleanly as is.

Thanks,
Hugh
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