[Intel-gfx] [RFC PATCH] drm/i915/userptr: Don't activate MMU notifier if no pages can be acquired

[Janusz Krzysztofik]

Hi Chris,

On Wednesday, February 19, 2020 1:17:12 PM CET Chris Wilson wrote:
> Quoting Janusz Krzysztofik (2020-02-19 12:09:44)
> > The purpose of userptr MMU notifier is to invalidate object pages as
> > soon as someone unpins them from memory.  While doing that,
> > obj->mm.lock is acquired.  If the notifier was called with obj->mm.lock
> > already held, a lockdep loop would be triggered.  That scenario is
> > believed to be possible in several cases, one of which is when the
> > userptr object is created from an mmap-offset mapping of another i915
> > GEM object.  This patch tries to address this case.
> > 
> > Even if creating a userptr object on an mmap-offset mapping succeeds,
> > trying to pin pages of the mapping in memory always fails because of
> > them having a VM_PFNMAP flag set.  However, the notifier can be
> > activated for a userptr object even before required pages are found
> > already pinned in memory, as soon as a worker expected to get missing
> > pages is scheduled successfully.  If the worker then fails to collect
> > the pages, it deactivates the notifier.  However, a time window exists
> > when the notifier can be called for an object even with no pages set
> > yet.
> 
> You mean something like
> https://patchwork.freedesktop.org/patch/275514/?series=54869&rev=2
> to avoid lockdep cross-contamination.

Yes, the purpose of both seems the same.  How could I help to get your 
preferred one merged?

Thanks,
Janusz


> -Chris
>