linux-kernel - Re: [RFC] Per file OOM badness

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Message-ID: <e511ba10-7032-36cc-f22c-2f6bb05f9f6e@daenzer.net>
Date:   Tue, 30 Jan 2018 12:34:18 +0100
From:   Michel Dänzer <michel@...nzer.net>
To:     christian.koenig@....com, Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <guro@...com>
Cc:     linux-mm@...ck.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, amd-gfx@...ts.freedesktop.org
Subject: Re: [RFC] Per file OOM badness

On 2018-01-30 12:28 PM, Christian König wrote:
> Am 30.01.2018 um 12:02 schrieb Michel Dänzer:
>> On 2018-01-30 11:40 AM, Christian König wrote:
>>> Am 30.01.2018 um 10:43 schrieb Michel Dänzer:
>>>> [SNIP]
>>>>> Would it be ok to hang onto potentially arbitrary mmget references
>>>>> essentially forever? If that's ok I think we can do your process based
>>>>> account (minus a few minor inaccuracies for shared stuff perhaps,
>>>>> but no
>>>>> one cares about that).
>>>> Honestly, I think you and Christian are overthinking this. Let's try
>>>> charging the memory to every process which shares a buffer, and go from
>>>> there.
>>> My problem is that this needs to be bullet prove.
>>>
>>> For example imagine an application which allocates a lot of BOs, then
>>> calls fork() and let the parent process die. The file descriptor lives
>>> on in the child process, but the memory is not accounted against the
>>> child.
>> What exactly are you referring to by "the file descriptor" here?
> 
> The file descriptor used to identify the connection to the driver. In
> other words our drm_file structure in the kernel.
> 
>> What happens to BO handles in general in this case? If both parent and
>> child process keep the same handle for the same BO, one of them
>> destroying the handle will result in the other one not being able to use
>> it anymore either, won't it?
> Correct.
> 
> That usage is actually not useful at all, but we already had
> applications which did exactly that by accident.
> 
> Not to mention that somebody could do it on purpose.

Can we just prevent child processes from using their parent's DRM file
descriptors altogether? Allowing it seems like a bad idea all around.


-- 
Earthling Michel Dänzer               |               http://www.amd.com
Libre software enthusiast             |             Mesa and X developer