[Intel-gfx] [PATCH v4] drm/i915: Clean up associated VMAs on context destruction

Daniel Vetter daniel at ffwll.ch
Tue Oct 6 05:15:56 PDT 2015

On Tue, Oct 06, 2015 at 10:48:28AM +0100, Tvrtko Ursulin wrote:
> On 06/10/15 10:34, Chris Wilson wrote:
> >On Tue, Oct 06, 2015 at 11:28:49AM +0200, Daniel Vetter wrote:
> >>On Tue, Oct 06, 2015 at 10:04:31AM +0100, Chris Wilson wrote:
> >>>On Tue, Oct 06, 2015 at 10:58:01AM +0200, Daniel Vetter wrote:
> >>>>On Mon, Oct 05, 2015 at 01:26:36PM +0100, Tvrtko Ursulin wrote:
> >>>>>From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> >>>>>
> >>>>>Prevent leaking VMAs and PPGTT VMs when objects are imported
> >>>>>via flink.
> >>>>>
> >>>>>Scenario is that any VMAs created by the importer will be left
> >>>>>dangling after the importer exits, or destroys the PPGTT context
> >>>>>with which they are associated.
> >>>>>
> >>>>>This is caused by object destruction not running when the
> >>>>>importer closes the buffer object handle due the reference held
> >>>>>by the exporter. This also leaks the VM since the VMA has a
> >>>>>reference on it.
> >>>>>
> >>>>>In practice these leaks can be observed by stopping and starting
> >>>>>the X server on a kernel with fbcon compiled in. Every time
> >>>>>X server exits another VMA will be leaked against the fbcon's
> >>>>>frame buffer object.
> >>>>>
> >>>>>Also on systems where flink buffer sharing is used extensively,
> >>>>>like Android, this leak has even more serious consequences.
> >>>>>
> >>>>>This version is takes a general approach from the  earlier work
> >>>>>by Rafael Barbalho (drm/i915: Clean-up PPGTT on context
> >>>>>destruction) and tries to incorporate the subsequent discussion
> >>>>>between Chris Wilson and Daniel Vetter.
> >>>>>
> >>>>>v2:
> >>>>>
> >>>>>Removed immediate cleanup on object retire - it was causing a
> >>>>>recursive VMA unbind via i915_gem_object_wait_rendering. And
> >>>>>it is in fact not even needed since by definition context
> >>>>>cleanup worker runs only after the last context reference has
> >>>>>been dropped, hence all VMAs against the VM belonging to the
> >>>>>context are already on the inactive list.
> >>>>>
> >>>>>v3:
> >>>>>
> >>>>>Previous version could deadlock since VMA unbind waits on any
> >>>>>rendering on an object to complete. Objects can be busy in a
> >>>>>different VM which would mean that the cleanup loop would do
> >>>>>the wait with the struct mutex held.
> >>>>>
> >>>>>This is an even simpler approach where we just unbind VMAs
> >>>>>without waiting since we know all VMAs belonging to this VM
> >>>>>are idle, and there is nothing in flight, at the point
> >>>>>context destructor runs.
> >>>>>
> >>>>>v4:
> >>>>>
> >>>>>Double underscore prefix for __915_vma_unbind_no_wait and a
> >>>>>commit message typo fix. (Michel Thierry)
> >>>>>
> >>>>>Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> >>>>>Testcase: igt/gem_ppgtt.c/flink-and-exit-vma-leak
> >>>>>Reviewed-by: Michel Thierry <michel.thierry at intel.com>
> >>>>>Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> >>>>>Cc: Chris Wilson <chris at chris-wilson.co.uk>
> >>>>>Cc: Rafael Barbalho <rafael.barbalho at intel.com>
> >>>>>Cc: Michel Thierry <michel.thierry at intel.com>
> >>>>
> >>>>Queued for -next, thanks for the patch.
> >>>
> >>>Please no, it's an awful patch and does not even fix the root cause of
> >>>the leak (that the vma are not closed when the handle is).
> >>
> >>It's a lose-lose situation for me as maintainer (and this holds in general
> >>really, not just for this patch):
> >>- Either I wield my considerable maintainer powers and force proper
> >>   solution, which will piss of a lot of people.
> >>- Or I just merge intermediate stuff and piss of another set of people
> >>   (including likely our all future selves because we're slowly digging a
> >>   tech debt grave).
> >>
> >>What I can't do with maintainer fu is force collaboration, I can only try
> >>to piss off everyone equally. And today the die rolled a "merge".
> >
> >Pity it didn't roll "what's the impact and where is the bugzilla?" :)
> There is this one:
> https://bugs.freedesktop.org/show_bug.cgi?id=87729
> And I could raise another one for leaking VMAs on X.org restart? :)

I added a small note to the patch that this isn't everything.
Daniel Vetter
Software Engineer, Intel Corporation

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