linux-kernel - Re: [PATCH] hugetlb: Acquire the i_mmap_lock before walking the prio

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Date:	Sat, 5 Dec 2009 12:37:00 +0000 (GMT)
From:	Hugh Dickins <hugh.dickins@...cali.co.uk>
To:	Mel Gorman <mel@....ul.ie>
cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] hugetlb: Acquire the i_mmap_lock before walking the
 prio_tree to unmap a page

On Wed, 2 Dec 2009, Mel Gorman wrote:
> On Wed, Dec 02, 2009 at 10:16:02PM +0000, Mel Gorman wrote:
> > On Wed, Dec 02, 2009 at 08:13:39PM +0000, Hugh Dickins wrote:
> > > 
> > > But the comment seems wrong to me: hugetlb_instantiation_mutex
> > > guards against concurrent hugetlb_fault()s; but the structure of
> > > the prio_tree shifts as vmas based on that inode are inserted into
> > > (mmap'ed) and removed from (munmap'ed) that tree (always while
> > > holding i_mmap_lock).  I don't see hugetlb_instantiation_mutex
> > > giving us any protection against this at present.
> > > 
> > 
> > You're right of course. I'll report without that nonsense included.
> > 
> 
> Actually, shouldn't the mmap_sem be protecting against concurrent mmap and
> munmap altering the tree? The comment is still bogus of course.

No, the mmap_sem can only protect against other threads sharing that
same mm: whereas the prio_tree can shift around according to concurrent
mmaps and munmaps of the same file in other mms.

Hugh
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