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Subject: Re: possible deadlock in __do_page_fault
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From: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
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Date: Tue, 22 Jan 2019 19:32:34 +0900
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To: Dmitry Vyukov <dvyukov@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Joel Fernandes <joel@joelfernandes.org>, Todd Kjos <tkjos@google.com>, Joel Fernandes <joelaf@google.com>, syzbot+a76129f18c89f3e2ddd4@syzkaller.appspotmail.com, Andi Kleen <ak@linux.intel.com>, Johannes Weiner <hannes@cmpxchg.org>, Jan Kara <jack@suse.cz>, Souptick Joarder <jrdr.linux@gmail.com>, LKML <linux-kernel@vger.kernel.org>, Linux-MM <linux-mm@kvack.org>, Matthew Wilcox <mawilcox@microsoft.com>, Mel Gorman <mgorman@techsingularity.net>, syzkaller-bugs <syzkaller-bugs@googlegroups.com>, =?UTF-8?Q?Arve_Hj=c3=b8nnev=c3=a5g?= <arve@android.com>, Todd Kjos <tkjos@android.com>, Martijn Coenen <maco@android.com>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>

On 2019/01/22 19:12, Dmitry Vyukov wrote:
> On Tue, Jan 22, 2019 at 11:02 AM Tetsuo Handa
> <penguin-kernel@i-love.sakura.ne.jp> wrote:
>>
>> On 2018/09/22 8:21, Andrew Morton wrote:
>>> On Thu, 20 Sep 2018 19:33:15 -0400 Joel Fernandes <joel@joelfernandes.org> wrote:
>>>
>>>> On Thu, Sep 20, 2018 at 5:12 PM Todd Kjos <tkjos@google.com> wrote:
>>>>>
>>>>> +Joel Fernandes
>>>>>
>>>>> On Thu, Sep 20, 2018 at 2:11 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>>>>>>
>>>>>>
>>>>>> Thanks.  Let's cc the ashmem folks.
>>>>>>
>>>>
>>>> This should be fixed by https://patchwork.kernel.org/patch/10572477/
>>>>
>>>> It has Neil Brown's Reviewed-by but looks like didn't yet appear in
>>>> anyone's tree, could Greg take this patch?
>>>
>>> All is well.  That went into mainline yesterday, with a cc:stable.
>>>
>>
>> This problem was not fixed at all.
> 
> There are at least 2 other open deadlocks involving ashmem:

Yes, they involve ashmem_shrink_scan() => {shmem|vfs}_fallocate() sequence.
This approach tries to eliminate this sequence.

> 
> https://syzkaller.appspot.com/bug?extid=148c2885d71194f18d28
> https://syzkaller.appspot.com/bug?extid=4b8b031b89e6b96c4b2e
> 
> Does this fix any of these too?

I need checks from ashmem folks whether this approach is possible/correct.
But you can ask syzbot to test this patch before ashmem folks respond.