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List:       linux-mm
Subject:    Re: [PATCH v3 03/14] mm, hugetlb: protect region tracking via newly introduced resv_map lock
From:       David Gibson <david () gibson ! dropbear ! id ! au>
Date:       2013-12-24 11:58:35
Message-ID: 20131224120012.GH12407 () voom ! fritz ! box
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On Mon, Dec 23, 2013 at 10:05:17AM +0900, Joonsoo Kim wrote:
> On Sun, Dec 22, 2013 at 12:58:19AM +1100, David Gibson wrote:
> > On Wed, Dec 18, 2013 at 03:53:49PM +0900, Joonsoo Kim wrote:
> > > There is a race condition if we map a same file on different processes.
> > > Region tracking is protected by mmap_sem and hugetlb_instantiation_mutex.
> > > When we do mmap, we don't grab a hugetlb_instantiation_mutex, but,
> > > grab a mmap_sem. This doesn't prevent other process to modify region
> > > structure, so it can be modified by two processes concurrently.
> > > 
> > > To solve this, I introduce a lock to resv_map and make region manipulation
> > > function grab a lock before they do actual work. This makes region
> > > tracking safe.
> > 
> > It's not clear to me if you're saying there is a list corruption race
> > bug in the existing code, or only that there will be if the
> > instantiation mutex goes away.
> 
> Hello,
> 
> The race exists in current code.
> Currently, region tracking is protected by either down_write(&mm->mmap_sem) or
> down_read(&mm->mmap_sem) + instantiation mutex. But if we map this hugetlbfs
> file to two different processes, holding a mmap_sem doesn't have any impact on
> the other process and concurrent access to data structure is possible.

Ouch.  In that case:

Acked-by: David Gibson <david@gibson.dropbear.id.au>

It would be really nice to add a testcase for this race to the
libhugetlbfs testsuite.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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