Re: [PATCH v2 8/8] drm/panthor: Add a GEM shrinker

[Boris Brezillon <boris.brezillon@collabora.com>]

On Mon, 2 Feb 2026 20:08:30 +0000
Akash Goel <akash.goel@arm.com> wrote:

> >> +bool panthor_gem_try_evict(struct drm_gem_object *obj,
> >> +			   struct ww_acquire_ctx *ticket)  
> > 
> > I think this could be static - I don't see any reference outside this
> > file (other than the header).

Will do.

> >   
> >> +{
> >> +	/*
> >> +	 * Track last locked entry for unwinding locks in error and
> >> +	 * success paths
> >> +	 */
> >> +	struct panthor_gem_object *bo = to_panthor_bo(obj);
> >> +	struct drm_gpuvm_bo *vm_bo, *last_locked = NULL;
> >> +	enum panthor_gem_reclaim_state old_state;
> >> +	int ret = 0;
> >> +
> >> +	/* To avoid potential lock ordering issue between bo_gpuva and
> >> +	 * mapping->i_mmap_rwsem, unmap the pages from CPU side before
> >> +	 * acquring the bo_gpuva lock. As the bo_resv lock is held, CPU
> >> +	 * page fault handler won't be able to map in the pages whilst
> >> +	 * eviction is in progress.
> >> +	 */
> >> +	drm_vma_node_unmap(&bo->base.vma_node, bo->base.dev->anon_inode->i_mapping);  
> > 
> > There might be an issue here - drm_gem_lru_scan() will have taken the
> > resv lock. drm_vma_node_unmap() could cause a callback to
> > panthor_vm_close(). If that ends up being the last reference to
> > bo->cmap.mmap_count then we'll deadlock attempting to aquire the resv
> > lock again.  
> 
> Actually drm_vma_node_unmap() would just invalidate the CPU PTEs.
> The CPU mapping won't be removed and so panthor_vm_close() won't get called.

Yep, that's also my understanding of drm_vma_node_unmap(): it kills the
relevant PTEs in the user VM, but leave the VMA active, so next time
there's an access, the fault handler will be called.

> 
> > 
> > I not 100% on that, and sadly it seems my test setup has died so I can't
> > test that out today.
> >  
> 
> We have tests that tries to trigger an evicition for a CPU mapped BO and 
> so far we didn't see a deadlock problem.

Actually, that's one of the very few tests I have in my igt branch [1],
and it was passing fine.

[1]https://gitlab.freedesktop.org/bbrezillon/igt-gpu-tools/-/commit/fc76934a5579767d2aabe787d40e38a17c3f4ea4#67d3c5d7df01192b03c20b43ad33249c663a95f5_80_97