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

[Neil Roberts]

Daniel Vetter <daniel at ffwll.ch> writes:

> 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.

Oh of course, the fault handler doesn’t check this. I’ve added a second
patch to make it check and posted it as a separate series here:

https://lists.freedesktop.org/archives/dri-devel/2021-February/298170.html

The two patches combined make the IGT test pass.

> 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.

I agree it would be great to have a debugfs option to trigger the purge.
I wonder if someone more involved in Panfrost would like to implement
this, because I am actually trying to work on VC4 and this is already
turning out to be quite a lot of yak shaving :) I’d also like to
implement the same debugfs option and IGT test for VC4.

Thanks for the feedback.

Regards,
- Neil