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

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Message-ID: <d8c8790c-8353-9d0d-416f-76967abc593c@gmail.com>
Date:   Tue, 30 Jan 2018 12:35:20 +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 11:48, Michel Dänzer wrote:
> On 2018-01-30 11:42 AM, Daniel Vetter wrote:
>> On Tue, Jan 30, 2018 at 10:43:10AM +0100, Michel Dänzer wrote:
>>> On 2018-01-30 10:31 AM, Daniel Vetter wrote:
>>>
>>>> I guess a good first order approximation would be if we simply charge any
>>>> newly allocated buffers to the process that created them, but that means
>>>> hanging onto lots of mm_struct pointers since we want to make sure we then
>>>> release those pages to the right mm again (since the process that drops
>>>> the last ref might be a totally different one, depending upon how the
>>>> buffers or DRM fd have been shared).
>>>>
>>>> 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.
>>
>> I'm not concerned about wrongly accounting shared buffers (they don't
>> matter), but imbalanced accounting. I.e. allocate a buffer in the client,
>> share it, but then the compositor drops the last reference.
> 
> I don't think the order matters. The memory is "uncharged" in each
> process when it drops its reference.

Daniel made a fair point about passing DRM fds between processes, though.

It's not a problem with how the fds are currently used, but somebody 
could do the following:

1. Create a DRM fd in process A, allocate lots of buffers.
2. Pass the fd to process B via some IPC mechanism.
3. Exit process A.

There needs to be some assurance that the BOs are accounted as belonging 
to process B in the end.

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