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Date: Mon, 15 Mar 2010 22:48:25 +0900
Message-ID: <28c262361003150648v59baf43cx829248c33b4ce607@mail.gmail.com>
Subject: Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped 
	anonymous pages
From: Minchan Kim <minchan.kim@gmail.com>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Mel Gorman <mel@csn.ul.ie>, Andrew Morton <akpm@linux-foundation.org>,
       Andrea Arcangeli <aarcange@redhat.com>,
       Christoph Lameter <cl@linux-foundation.org>,
       Adam Litke <agl@us.ibm.com>, Avi Kivity <avi@redhat.com>,
       David Rientjes <rientjes@google.com>,
       KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
       Rik van Riel <riel@redhat.com>, linux-kernel@vger.kernel.org,
       linux-mm@kvack.org
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On Mon, Mar 15, 2010 at 4:09 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@jp.fujitsu.com> wrote:
> On Mon, 15 Mar 2010 15:44:59 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
>
>> On Mon, 15 Mar 2010 15:28:15 +0900
>> Minchan Kim <minchan.kim@gmail.com> wrote:
>>
>> > On Mon, Mar 15, 2010 at 2:34 PM, KAMEZAWA Hiroyuki
>> > <kamezawa.hiroyu@jp.fujitsu.com> wrote:
>> > > On Mon, 15 Mar 2010 09:28:08 +0900
>> > > Minchan Kim <minchan.kim@gmail.com> wrote:
>
>> > I think above scenario make error "use-after-free", again.
>> > What prevent above scenario?
>> >
>> I think this patch is not complete.
>> I guess this patch in [1/11] is trigger for the race.
>> ==
>> +
>> +     /* Drop an anon_vma reference if we took one */
>> +     if (anon_vma && atomic_dec_and_lock(&anon_vma->migrate_refcount, &anon_vma->lock)) {
>> +             int empty = list_empty(&anon_vma->head);
>> +             spin_unlock(&anon_vma->lock);
>> +             if (empty)
>> +                     anon_vma_free(anon_vma);
>> +     }
>> ==
>> If my understainding in above is correct, this "modify" freed anon_vma.
>> Then, use-after-free happens. (In old implementation, there are no refcnt,
>> so, there is no use-after-free ops.)
>>
> Sorry, about above, my understanding was wrong. anon_vma->lock is modifed even
> in old code. Sorry for noise.

Nope.  Such your kindness always helps and cheer up others people.
In addition, give others good time to consider seriously something.

Thanks, Kame.


-- 
Kind regards,
Minchan Kim