linux-kernel - Re: [PATCH v2] hugetlb: fix race condition in hugetlb

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Message-ID: <4F7887A5.3060700@tilera.com>
Date:	Sun, 1 Apr 2012 12:51:49 -0400
From:	Chris Metcalf <cmetcalf@...era.com>
To:	Hillf Danton <dhillf@...il.com>
CC:	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH v2] hugetlb: fix race condition in hugetlb_fault()

On 4/1/2012 8:10 AM, Hillf Danton wrote:
> On Sat, Mar 31, 2012 at 4:07 AM, Chris Metcalf <cmetcalf@...era.com> wrote:
>> The race is as follows.  Suppose a multi-threaded task forks a new
>> process, thus bumping up the ref count on all the pages.  While the fork
>> is occurring (and thus we have marked all the PTEs as read-only), another
>> thread in the original process tries to write to a huge page, taking an
>> access violation from the write-protect and calling hugetlb_cow().  Now,
>> suppose the fork() fails.  It will undo the COW and decrement the ref
>> count on the pages, so the ref count on the huge page drops back to 1.
>> Meanwhile hugetlb_cow() also decrements the ref count by one on the
>> original page, since the original address space doesn't need it any more,
>> having copied a new page to replace the original page.  This leaves the
>> ref count at zero, and when we call unlock_page(), we panic.
>>
>> The solution is to take an extra reference to the page while we are
>> holding the lock on it.
>>
> If the following chart matches the above description,
>
> [...]
>
> would you please spin with description refreshed?

Done, and thanks!  I added your timeline chart to my description; I figure
no harm in having it both ways.

>> Cc: stable@...nel.org
> Let Andrew do the stable work, ok?

Fair point.  I'm used to adding the Cc myself for things I push through the
arch/tile tree.  This of course does make more sense to go through Andrew,
so I'll remove it.

-- 
Chris Metcalf, Tilera Corp.
http://www.tilera.com

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