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

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Date:   Tue, 30 Jan 2018 12:36:37 +0100
From:   Nicolai Hähnle <nhaehnle@...il.com>
To:     Michel Dänzer <michel@...nzer.net>,
        christian.koenig@....com, Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <guro@...com>
Cc:     linux-mm@...ck.org, amd-gfx@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [RFC] Per file OOM badness

On 30.01.2018 12:34, Michel Dänzer wrote:
> 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.

Existing protocols pass DRM fds between processes though, don't they?

Not child processes perhaps, but special-casing that seems like awful 
design.

Cheers,
Nicolai
-- 
Lerne, wie die Welt wirklich ist,
Aber vergiss niemals, wie sie sein sollte.