Re: [PATCH v3 8/9] drm/panthor: Track the number of mmap on a BO

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

On Wed, 11 Feb 2026 16:08:23 +0000
Steven Price <steven.price@arm.com> wrote:

> On 11/02/2026 08:03, Boris Brezillon wrote:
> > This will be used to order things by reclaimability.
> > 
> > v2:
> > - Fix refcounting
> > 
> > v3:
> > - Fix refcounting (again)
> > 
> > Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>  
> 
> I think we've got the refcounting correct this time, one minor comment
> below but either way:
> 
> Reviewed-by: Steven Price <steven.price@arm.com>
> 
> > ---
> >  drivers/gpu/drm/panthor/panthor_gem.c | 45 +++++++++++++++++++++++++--
> >  drivers/gpu/drm/panthor/panthor_gem.h |  3 ++
> >  2 files changed, 46 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/panthor/panthor_gem.c b/drivers/gpu/drm/panthor/panthor_gem.c
> > index 8905042b856c..e46bfc4f2063 100644
> > --- a/drivers/gpu/drm/panthor/panthor_gem.c
> > +++ b/drivers/gpu/drm/panthor/panthor_gem.c
> > @@ -491,6 +491,7 @@ static void panthor_gem_print_info(struct drm_printer *p, unsigned int indent,
> >  	drm_printf_indent(p, indent, "vmap_use_count=%u\n",
> >  			  refcount_read(&bo->cmap.vaddr_use_count));
> >  	drm_printf_indent(p, indent, "vaddr=%p\n", bo->cmap.vaddr);
> > +	drm_printf_indent(p, indent, "mmap_count=%u\n", refcount_read(&bo->cmap.mmap_count));
> >  }
> >  
> >  static int panthor_gem_pin_locked(struct drm_gem_object *obj)
> > @@ -606,6 +607,13 @@ static int panthor_gem_mmap(struct drm_gem_object *obj, struct vm_area_struct *v
> >  	if (is_cow_mapping(vma->vm_flags))
> >  		return -EINVAL;
> >  
> > +	if (!refcount_inc_not_zero(&bo->cmap.mmap_count)) {
> > +		dma_resv_lock(obj->resv, NULL);
> > +		if (!refcount_inc_not_zero(&bo->cmap.mmap_count))
> > +			refcount_set(&bo->cmap.mmap_count, 1);  
> 
> It could make sense to put a call to
> panthor_gem_update_reclaim_state_locked() in here as we could transition
> from UNUSED to MMAPPED. Things should work out due to the call in
> panthor_gem_try_evict(), but the shrinker will have to work harder.

Sure, I'll add that to the next commit.

> 
> Thanks,
> Steve
> 
> > +		dma_resv_unlock(obj->resv);
> > +	}
> > +
> >  	vm_flags_set(vma, VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP);
> >  	vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
> >  	if (should_map_wc(bo))
> > @@ -732,10 +740,43 @@ static vm_fault_t panthor_gem_fault(struct vm_fault *vmf)
> >  	return blocking_page_setup(vmf, bo, page_offset, true);
> >  }
> >  
> > +static void panthor_gem_vm_open(struct vm_area_struct *vma)
> > +{
> > +	struct panthor_gem_object *bo = to_panthor_bo(vma->vm_private_data);
> > +
> > +	/* mmap_count must have been incremented at mmap time, so it can't be
> > +	 * zero here.
> > +	 */
> > +	if (!drm_gem_is_imported(&bo->base))
> > +		drm_WARN_ON(bo->base.dev, !refcount_inc_not_zero(&bo->cmap.mmap_count));
> > +
> > +	drm_gem_vm_open(vma);
> > +}
> > +
> > +static void panthor_gem_vm_close(struct vm_area_struct *vma)
> > +{
> > +	struct panthor_gem_object *bo = to_panthor_bo(vma->vm_private_data);
> > +
> > +	if (drm_gem_is_imported(&bo->base))
> > +		goto out;
> > +
> > +	if (refcount_dec_not_one(&bo->cmap.mmap_count))
> > +		goto out;
> > +
> > +	dma_resv_lock(bo->base.resv, NULL);
> > +	if (refcount_dec_and_test(&bo->cmap.mmap_count)) {
> > +		/* Nothing to do, pages are reclaimed lazily. */
> > +	}
> > +	dma_resv_unlock(bo->base.resv);
> > +
> > +out:
> > +	drm_gem_object_put(&bo->base);
> > +}
> > +
> >  const struct vm_operations_struct panthor_gem_vm_ops = {
> >  	.fault = panthor_gem_fault,
> > -	.open = drm_gem_vm_open,
> > -	.close = drm_gem_vm_close,
> > +	.open = panthor_gem_vm_open,
> > +	.close = panthor_gem_vm_close,
> >  };
> >  
> >  static const struct drm_gem_object_funcs panthor_gem_funcs = {
> > diff --git a/drivers/gpu/drm/panthor/panthor_gem.h b/drivers/gpu/drm/panthor/panthor_gem.h
> > index b66478c9590c..c0a18dca732c 100644
> > --- a/drivers/gpu/drm/panthor/panthor_gem.h
> > +++ b/drivers/gpu/drm/panthor/panthor_gem.h
> > @@ -80,6 +80,9 @@ struct panthor_gem_cpu_map {
> >  
> >  	/** @vaddr_use_count: Number of active vmap() requests on this GEM */
> >  	refcount_t vaddr_use_count;
> > +
> > +	/** @mmap_count: Number of active mmap() requests on this GEM */
> > +	refcount_t mmap_count;
> >  };
> >  
> >  /**  
>