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
