[PATCH] drm/shmem-helper: Don't remove the offset in vm_area_struct pgoff

[Daniel Vetter]

On Tue, Feb 23, 2021 at 1:42 PM Neil Roberts <nroberts at igalia.com> wrote:
>
> Daniel Vetter <daniel at ffwll.ch> writes:
>
> > Yeah plus Cc: stable for backporting and I think an igt or similar for
> > panfrost to check this works correctly would be pretty good too. Since
> > if it took us over 1 year to notice this bug it's pretty clear that
> > normal testing doesn't catch this. So very likely we'll break this
> > again.
>
> I made the IGT test below which seems to reproduce the bug. However, the
> kernel patch doesn’t fix it, so maybe there is something more subtle
> going on.
>
> https://gitlab.freedesktop.org/nroberts/igt-gpu-tools/-/commits/panfrost-purgemap/

drm_gem_shmem_fault() does not seem to check for purged objects at all.

No idea how this works, or if it ever worked, but yeah something is
clearly still busted.

Definitely a good idae to have an igt. btw to make that faster you can
either use the vm_drop_caches file from proc (it's a bit a hammer), or
what I recommend: Have a dedicated debugfs file to only drop
everything from your shrinker. That's much quicker and  controlled.
See e.g. ttm_tt_debugfs_shrink from d4bd7776a7ac ("drm/ttm: rework
ttm_tt page limit v4") which recently landed in drm-misc-next.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch