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

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Message-ID: <CAJd=RBCem0hw8w1ehNZnzb6OMqn1xsqT9yczgDag0ydp9mavCw@mail.gmail.com>
Date:	Sat, 19 Nov 2011 11:26:59 +0800
From:	Hillf Danton <dhillf@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Michal Hocko <mhocko@...e.cz>,
	Andrea Arcangeli <aarcange@...hat.com>,
	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 Sat, Nov 19, 2011 at 3:39 AM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Fri, 18 Nov 2011 17:11:28 +0100
> Michal Hocko <mhocko@...e.cz> 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.
>>
>> Reviewed-by: Michal Hocko <mhocko@...e.cz>
>
> So we need a new changelog.  How does this look?
>
Thanks Andrew and Michal:)

Best regards
Hillf
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