[PATCH v2 6/7] drm/panfrost: Add support for GPU heap allocations

[Steven Price]

On 25/07/2019 15:59, Steven Price wrote:
[...]
> It would appear that in the following call sgt==NULL:
>> 	ret = sg_alloc_table_from_pages(sgt, pages + page_offset,
>> 					NUM_FAULT_PAGES, 0, SZ_2M, GFP_KERNEL);
> 
> Which means we've ended up with a BO with bo->sgt==NULL, bo->pages set
> and bo->is_heap=true. My understanding is this should be impossible.
> 
> I haven't yet figured out how this happens - it seems to be just before
> termination, so it might be a race with cleanup?

That was a red herring - it's partly my test case doing something a bit
weird. This crash is caused by doing an mmap of a HEAP object before any
fault has occurred.

drm_gem_shmem_mmap() calls drm_gem_shmem_get_pages() which will populate
bo->base.pages (even if bo->is_heap).

Either we should prevent mapping of HEAP objects, or alternatively
bo->base.pages could be allocated upfront instead of during the first
fault. My preference would be allocating it upfront because optimising
for the case of a HEAP BO which isn't used seems a bit weird. Although
there's still the question of exactly what the behaviour should be of
accessing through the CPU pages which haven't been allocated yet.

Also shmem->pages_use_count needs incrementing to stop
drm_gem_shmem_get_pages() replacing bo->base.pages. I haven't tested
what happens if you mmap *after* the first fault.

Steve