Re: [PATCH v2 2/5] drm/panthor: Fix iova_mapped_as_huge_page() for imported BOs

[Steven Price <steven.price@arm.com>]

On 24/09/2026 12:04, Boris Brezillon wrote:
> Imported BOs have no backing.pages array allocated, leading to a NULL
> deref when iova_mapped_as_huge_page() gets called on them.
> 
> Implement this check through and sgt walk to reach the position of the
NIT:                           ^^^
s/and/an/

> sgt targeted by a VA, and check that the DMA address is properly aligned
> and the size remaining in the SG entry is bigger than a huge page.
> This is basically matching the logic in vm_map_pages(), with get_pgsize()
> being replaced by a simpler test, because we don't care about the pgcount
> info.
> 
> Reported-by: Akash Goel <akash.goel@arm.com>
> Fixes: 8e7460eac786 ("drm/panthor: Support partial unmaps of huge pages")
> Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
> Reviewed-by: Akash Goel <akash.goel@arm.com>
> Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>

Reviewed-by: Steven Price <stevne.price@arm.com>

Although there's a minor style issue below if you do respin (see below).

> ---
>  drivers/gpu/drm/panthor/panthor_mmu.c | 44 ++++++++++++++++++++++++++++-------
>  1 file changed, 36 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_mmu.c b/drivers/gpu/drm/panthor/panthor_mmu.c
> index b0a7033480e6..6cef954e2cba 100644
> --- a/drivers/gpu/drm/panthor/panthor_mmu.c
> +++ b/drivers/gpu/drm/panthor/panthor_mmu.c
> @@ -2299,7 +2299,6 @@ iova_mapped_as_huge_page(struct drm_gpuva *mapping, u64 va)
>  {
>  	struct panthor_gem_object *bo = to_panthor_bo(mapping->gem.obj);
>  	u64 aligned_va = ALIGN_DOWN(va, SZ_2M);
> -	const struct page *pg;
>  	pgoff_t bo_offset;
>  
>  	/* If the 2M-aligned VA is outside the mapping being tested, we know
> @@ -2309,16 +2308,45 @@ iova_mapped_as_huge_page(struct drm_gpuva *mapping, u64 va)
>  		return false;
>  
>  	bo_offset = aligned_va - mapping->va.addr + mapping->gem.offset;
> -	pg = bo->backing.pages[bo_offset >> PAGE_SHIFT];
>  
> -	/* In case of shmem backing, we know we can only have a huge mapping
> -	 * if the bo_offset is 2M aligned, meaning we can skip the folio size
> -	 * check if it's not the case.
> -	 */
> -	if (!IS_ALIGNED(bo_offset, SZ_2M))
> +	if (drm_gem_is_imported(&bo->base)) {
> +		struct sg_table *sgt = bo->dmap.sgt;
> +		struct scatterlist *sgl;
> +		unsigned int count;
> +
> +		/* If this is an imported BO, we have to walk the SGT and
> +		 * check the dma address/size alignment to determine if it's
> +		 * a huge map or not.
> +		 */
> +		for_each_sgtable_dma_sg(sgt, sgl, count) {
> +			size_t len = sg_dma_len(sgl);
> +			dma_addr_t daddr;
> +
> +			if (len <= bo_offset) {
> +				bo_offset -= len;
> +				continue;
> +			}
> +
> +			len -= bo_offset;
> +			daddr = sg_dma_address(sgl) + bo_offset;
> +
> +			return IS_ALIGNED(daddr, SZ_2M) &&
> +			       len >= SZ_2M;
> +		}
> +
>  		return false;
> +	} else {

This 'else' branch isn't needed - since the above branch returns early.
Dropping the below back out of the braces would make the diff easier to
read - effectively the drm_gem_is_imported() branch is just added and
the rest of the code is left as is.

Thanks,
Steve

> +		const struct page *pg = bo->backing.pages[bo_offset >> PAGE_SHIFT];
>  
> -	return folio_size(page_folio(pg)) >= SZ_2M;
> +		/* In case of shmem backing, we know we can only have a huge mapping
> +		 * if the bo_offset is 2M aligned, meaning we can skip the folio size
> +		 * check if it's not the case.
> +		 */
> +		if (!IS_ALIGNED(bo_offset, SZ_2M))
> +			return false;
> +
> +		return folio_size(page_folio(pg)) >= SZ_2M;
> +	}
>  }
>  
>  static void
>