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

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Date:	Fri, 18 Nov 2011 17:11:28 +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 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>

> 
> Thanks,
> Hillf
> 
> 	/*
> 	 * Retake the page_table_lock to check for racing updates
> 	 * before the page tables are altered
> 	 */
> 	spin_lock(&mm->page_table_lock);
> 	ptep = huge_pte_offset(mm, address & huge_page_mask(h));
> 	if (likely(pte_same(huge_ptep_get(ptep), pte))) {
> 		/* Break COW */
> 
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-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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