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

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Date:   Tue, 30 Jan 2018 12:42:20 +0100
From:   Michel Dänzer <michel@...nzer.net>
To:     Nicolai Hähnle <nhaehnle@...il.com>,
        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 12:36 PM, Nicolai Hähnle wrote:
> 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.

Fair enough.

Can we disallow passing DRM file descriptors which have any buffers
allocated? :)


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