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

[Chris Wilson]

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.
-Chris