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

[Daniel Vetter]

On Tue, Feb 23, 2021 at 4:59 PM Neil Roberts <nroberts at igalia.com> wrote:
>
> 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.

If we push the shrinker setup into the helpers (this means we
minimally need an lru, and probably more reasonable locking that shmem
helpers uses right now) then we could have one debugfs file for all
drivers supporting purgeable objects. Could then even share some of
the igt, only the ioctl code would need to be driver specific.

It's a bit of work though.
-Daniel

> Thanks for the feedback.
>
> Regards,
> - Neil



-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch