linux-kernel - Re: [PATCH v3 03/14] mm, hugetlb: protect region tracking via newly introduced resv

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Date:	Mon, 23 Dec 2013 10:05:17 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	David Gibson <david@...son.dropbear.id.au>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Michal Hocko <mhocko@...e.cz>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Hugh Dickins <hughd@...gle.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Wanpeng Li <liwanp@...ux.vnet.ibm.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Hillf Danton <dhillf@...il.com>
Subject: Re: [PATCH v3 03/14] mm, hugetlb: protect region tracking via newly
 introduced resv_map lock

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.

Thanks.
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