linux-kernel - Re: [PATCH] hugetlb: detect race if fail to COW

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Message-ID: <20111121122303.GA13594@tiehlicka.suse.cz>
Date:	Mon, 21 Nov 2011 13:23:03 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Hillf Danton <dhillf@...il.com>
Cc:	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <jweiner@...hat.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] hugetlb: detect race if fail to COW

On Fri 18-11-11 17:11:28, Michal Hocko wrote:
> On Fri 18-11-11 23:23:12, Hillf Danton wrote:
> > On Fri, Nov 18, 2011 at 11:07 PM, Michal Hocko <mhocko@...e.cz> wrote:
> > > On Fri 18-11-11 22:04:37, Hillf Danton wrote:
> > >> In the error path that we fail to allocate new huge page, before try again, we
> > >> have to check race since page_table_lock is re-acquired.
> > >
> > > I do not think we can race here because we are serialized by
> > > hugetlb_instantiation_mutex AFAIU. Without this lock, however, we could
> > > fall into avoidcopy and shortcut despite the fact that other thread has
> > > already did the job.
> > >
> > > The mutex usage is not obvious in hugetlb_cow so maybe we want to be
> > > explicit about it (either a comment or do the recheck).
> > >
> > 
> > Then the following check is unnecessary, no?
> 
> Hmm, thinking about it some more, I guess we have to recheck because we
> can still race with page migration. So we need you patch.

OK, so looked at it again and we cannot race with page migration because
the page is locked (by unmap_and_move_*page) migration and we have the
old page locked here as well (hugetlb_fault).

Or am I missing something?

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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