linux-kernel - Re: [PATCH] mm: hugetlbfs: Close race during teardown of hugetlbfs shared page tables V2 (resend)

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Date:	Fri, 20 Jul 2012 16:40:42 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Linux-MM <linux-mm@...ck.org>, Hugh Dickins <hughd@...gle.com>,
	David Gibson <david@...son.dropbear.id.au>,
	Ken Chen <kenchen@...gle.com>,
	Cong Wang <xiyou.wangcong@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: hugetlbfs: Close race during teardown of hugetlbfs
 shared page tables V2 (resend)

On Fri 20-07-12 15:37:53, Mel Gorman wrote:
> On Fri, Jul 20, 2012 at 04:29:20PM +0200, Michal Hocko wrote:
> > > <SNIP>
> > > 
> > > Signed-off-by: Mel Gorman <mgorman@...e.de>
> > 
> > Yes this looks correct. mmap_sem will make sure that unmap_vmas and
> > free_pgtables are executed atomicaly wrt. huge_pmd_share so it doesn't
> > see non-NULL spte on the way out.
> 
> Yes.
> 
> > I am just wondering whether we need
> > the page_table_lock as well. It is not harmful but I guess we can drop
> > it because both exit_mmap and shmdt are not taking it and mmap_sem is
> > sufficient for them.
> 
> While it is true that we don't *really* need page_table_lock here, we are
> still updating page tables and it's in line with the the ordinary locking
> rules.  There are other cases in hugetlb.c where we do pte_same() checks even
> though we are protected from the related races by the instantiation_mutex.
> 
> page_table_lock is actually a bit useless for shared page tables. If shared
> page tables were every to be a general thing then I think we'd have to
> revisit how PTE update locking is done but I doubt anyone wants to dive
> down that rat-hole.
> 
> For now, I'm going to keep taking it even if strictly speaking it's not
> necessary.

Fair enough

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