Re: [RFC] Per file OOM badness

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Jan 30, 2018 at 10:43:10AM +0100, Michel Dänzer wrote:
> On 2018-01-30 10:31 AM, Daniel Vetter wrote:
> > On Wed, Jan 24, 2018 at 01:11:09PM +0100, Christian König wrote:
> >> Am 24.01.2018 um 12:50 schrieb Michal Hocko:
> >>> On Wed 24-01-18 12:23:10, Michel Dänzer wrote:
> >>>> On 2018-01-24 12:01 PM, Michal Hocko wrote:
> >>>>> On Wed 24-01-18 11:27:15, Michel Dänzer wrote:
> >>> [...]
> >>>>>> 2. If the OOM killer kills a process which is sharing BOs with another
> >>>>>> process, this should result in the other process dropping its references
> >>>>>> to the BOs as well, at which point the memory is released.
> >>>>> OK. How exactly are those BOs mapped to the userspace?
> >>>> I'm not sure what you're asking. Userspace mostly uses a GEM handle to
> >>>> refer to a BO. There can also be userspace CPU mappings of the BO's
> >>>> memory, but userspace doesn't need CPU mappings for all BOs and only
> >>>> creates them as needed.
> >>> OK, I guess you have to bear with me some more. This whole stack is a
> >>> complete uknonwn. I am mostly after finding a boundary where you can
> >>> charge the allocated memory to the process so that the oom killer can
> >>> consider it. Is there anything like that? Except for the proposed file
> >>> handle hack?
> >>
> >> Not that I knew of.
> >>
> >> As I said before we need some kind of callback that a process now starts to
> >> use a file descriptor, but without anything from that file descriptor mapped
> >> into the address space.
> > 
> > For more context: With DRI3 and wayland the compositor opens the DRM fd
> > and then passes it to the client, which then starts allocating stuff. That
> > makes book-keeping rather annoying.
> 
> Actually, what you're describing is only true for the buffers shared by
> an X server with an X11 compositor. For the actual applications, the
> buffers are created on the client side and then shared with the X server
> / Wayland compositor.
> 
> Anyway, it doesn't really matter. In all cases, the buffers are actually
> used by all parties that are sharing them, so charging the memory to all
> of them is perfectly appropriate.
> 
> 
> > 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.

If we store the mm_struct pointer in drm_gem_object, we don't need any
callback from the vfs when fds are shared or anything like that. We can
simply account any newly allocated buffers to the current->mm, and then
store that later for dropping the account for when the gem obj is
released. This would entirely ignore any complications with shared
buffers, which I think we can do because even when we pass the DRM fd to a
different process, the actual buffer allocations are not passed around
like that for private buffers. And private buffers are the only ones that
really matter.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch