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

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Message-ID: <20091202221947.GB26702@csn.ul.ie>
Date:	Wed, 2 Dec 2009 22:19:47 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Hugh Dickins <hugh.dickins@...cali.co.uk>
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, Dec 02, 2009 at 10:16:02PM +0000, Mel Gorman wrote:
> On Wed, Dec 02, 2009 at 08:13:39PM +0000, Hugh Dickins wrote:
> > On Wed, 2 Dec 2009, Mel Gorman wrote:
> > 
> > > When the owner of a mapping fails COW because a child process is holding a
> > > reference and no pages are available, the children VMAs are walked and the
> > > page is unmapped. The i_mmap_lock is taken for the unmapping of the page but
> > > not the walking of the prio_tree. In theory, that tree could be changing
> > > while the lock is released although in practice it is protected by the
> > > hugetlb_instantiation_mutex. This patch takes the i_mmap_lock properly for
> > > the duration of the prio_tree walk in case the hugetlb_instantiation_mutex
> > > ever goes away.
> > > 
> > > [hugh.dickins@...cali.co.uk: Spotted the problem in the first place]
> > > Signed-off-by: Mel Gorman <mel@....ul.ie>
> > 
> > The patch looks good - thanks for taking care of that, Mel.
> > 
> > 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.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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