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Subject: Re: [PATCH] mm,oom: Bring OOM notifier callbacks to outside of OOM
 killer.
To:     Michal Hocko <mhocko@kernel.org>
CC:     Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
        <linux-mm@kvack.org>, <rientjes@google.com>,
        <akpm@linux-foundation.org>, <linux-kernel@vger.kernel.org>
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From:   peter enderborg <peter.enderborg@sony.com>
Message-ID: <9a14d554-6470-e0d6-19cc-1ecec17a47c7@sony.com>
Date:   Mon, 25 Jun 2018 16:04:04 +0200
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On 06/25/2018 03:07 PM, Michal Hocko wrote:

> On Mon 25-06-18 15:03:40, peter enderborg wrote:
>> On 06/20/2018 01:55 PM, Michal Hocko wrote:
>>> On Wed 20-06-18 20:20:38, Tetsuo Handa wrote:
>>>> Sleeping with oom_lock held can cause AB-BA lockup bug because
>>>> __alloc_pages_may_oom() does not wait for oom_lock. Since
>>>> blocking_notifier_call_chain() in out_of_memory() might sleep, sleeping
>>>> with oom_lock held is currently an unavoidable problem.
>>> Could you be more specific about the potential deadlock? Sleeping while
>>> holding oom lock is certainly not nice but I do not see how that would
>>> result in a deadlock assuming that the sleeping context doesn't sleep on
>>> the memory allocation obviously.
>> It is a mutex you are supposed to be able to sleep.  It's even exported.
> What do you mean? oom_lock is certainly not exported for general use. It
> is not local to oom_killer.c just because it is needed in other _mm_
> code.
>  

It  is in the oom.h file include/linux/oom.h, if it that sensitive it should
be in mm/ and a documented note about the special rules. It is only used
in drivers/tty/sysrq.c and that be replaced by a help function in mm that
do the  oom stuff.


>>>> As a preparation for not to sleep with oom_lock held, this patch brings
>>>> OOM notifier callbacks to outside of OOM killer, with two small behavior
>>>> changes explained below.
>>> Can we just eliminate this ugliness and remove it altogether? We do not
>>> have that many notifiers. Is there anything fundamental that would
>>> prevent us from moving them to shrinkers instead?
>> @Hocko Do you remember the lowmemorykiller from android? Some things
>> might not be the right thing for shrinkers.
> Just that lmk did it wrong doesn't mean others have to follow.
>
If all you have is a hammer, everything looks like a nail. (I don’t argument that it was right)
But if you don’t have a way to interact with the memory system we will get attempts like lmk. 
Oom notifiers and vmpressure is for this task better than shrinkers.