Re: [PATCH v3 1/1] drm/panthor: Support partial unmaps of huge pages

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

On Sat, 13 Dec 2025 19:08:33 +0000
Adrián Larumbe <adrian.larumbe@collabora.com> wrote:

> Commit 33729a5fc0ca ("iommu/io-pgtable-arm: Remove split on unmap
> behavior") did away with the treatment of partial unmaps of huge IOPTEs.
> 
> In the case of Panthor, that means an attempt to run a VM_BIND unmap
> operation on a memory region whose start address and size aren't 2MiB
> aligned, in the event it intersects with a huge page, would lead to ARM
> IOMMU management code to fail and a warning being raised.
> 
> Presently, and for lack of a better alternative, it's best to have
> Panthor handle partial unmaps at the driver level, by unmapping entire
> huge pages and remapping the difference between them and the requested
> unmap region.
> 
> This could change in the future when the VM_BIND uAPI is expanded to
> enforce huge page alignment and map/unmap operational constraints that
> render this code unnecessary.
> 
> Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
> ---
>  drivers/gpu/drm/panthor/panthor_mmu.c | 66 +++++++++++++++++++++++++++
>  1 file changed, 66 insertions(+)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_mmu.c b/drivers/gpu/drm/panthor/panthor_mmu.c
> index 183da30fa500..f11340a7f59e 100644
> --- a/drivers/gpu/drm/panthor/panthor_mmu.c
> +++ b/drivers/gpu/drm/panthor/panthor_mmu.c
> @@ -2110,6 +2110,44 @@ static int panthor_gpuva_sm_step_map(struct drm_gpuva_op *op, void *priv)
>  	return 0;
>  }
>  
> +static bool
> +iova_mapped_as_huge_page(const struct panthor_vma *vma, u64 addr)
> +{
> +	const struct page *pg;
> +	pgoff_t bo_offset;
> +
> +	bo_offset = addr - vma->base.va.addr + vma->base.gem.offset;
> +	pg = to_panthor_bo(vma->base.gem.obj)->base.pages[bo_offset >> PAGE_SHIFT];
> +
> +	return (folio_size(page_folio(pg)) >= SZ_2M);
> +}
> +
> +static void
> +get_map_unmap_intervals(const struct drm_gpuva_op_remap *op,
> +			const struct panthor_vma *unmap_vma,
> +			u64 *unmap_start, u64 *unmap_range)
> +{
> +	u64 aligned_unmap_start, aligned_unmap_end, unmap_end;
> +
> +	drm_gpuva_op_remap_to_unmap_range(op, unmap_start, unmap_range);
> +	unmap_end = *unmap_start + *unmap_range;
> +
> +	aligned_unmap_start = ALIGN_DOWN(*unmap_start, SZ_2M);
> +	if (op->prev && aligned_unmap_start < *unmap_start &&
> +	    op->prev->va.addr <= aligned_unmap_start &&
> +	    iova_mapped_as_huge_page(unmap_vma, *unmap_start)) {
> +		*unmap_range += *unmap_start - aligned_unmap_start;
> +		*unmap_start = aligned_unmap_start;
> +	}
> +
> +	aligned_unmap_end = ALIGN(unmap_end, SZ_2M);
> +	if (op->next && aligned_unmap_end > unmap_end &&
> +	    op->next->va.addr + op->next->va.range >= aligned_unmap_end &&
> +	    iova_mapped_as_huge_page(unmap_vma, unmap_end - 1)) {
> +		*unmap_range += aligned_unmap_end - unmap_end;
> +	}
> +}
> +
>  static int panthor_gpuva_sm_step_remap(struct drm_gpuva_op *op,
>  				       void *priv)
>  {
> @@ -2121,16 +2159,44 @@ static int panthor_gpuva_sm_step_remap(struct drm_gpuva_op *op,
>  	int ret;
>  
>  	drm_gpuva_op_remap_to_unmap_range(&op->remap, &unmap_start, &unmap_range);
> +
> +	/*
> +	 * ARM IOMMU page table management code disallows partial unmaps of huge pages,
> +	 * so when a partial unmap is requested, we must first unmap the entire huge
> +	 * page and then remap the difference between the huge page minus the requested
> +	 * unmap region. Calculating the right offsets and ranges for the different unmap
> +	 * and map operations is the responsibility of the following function.
> +	 */
> +	get_map_unmap_intervals(&op->remap, unmap_vma, &unmap_start, &unmap_range);
> +
>  	ret = panthor_vm_unmap_pages(vm, unmap_start, unmap_range);

This needs to be rebased on drm-misc-next: the panthor_vm_unmap_pages()
prototype has changed there.

>  	if (ret)
>  		return ret;
>  
>  	if (op->remap.prev) {
> +		ret = panthor_vm_map_pages(vm, unmap_start,
> +					   flags_to_prot(unmap_vma->flags),
> +					   to_drm_gem_shmem_obj(op->remap.prev->gem.obj)->sgt,
> +					   op->remap.prev->gem.offset +
> +					   (unmap_start - op->remap.prev->va.addr),
> +					   op->remap.prev->va.addr + op->remap.prev->va.range -
> +					   unmap_start);
> +		if (ret)
> +			return ret;
> +
>  		prev_vma = panthor_vm_op_ctx_get_vma(op_ctx);
>  		panthor_vma_init(prev_vma, unmap_vma->flags);
>  	}
>  
>  	if (op->remap.next) {
> +		ret = panthor_vm_map_pages(vm, op->remap.next->va.addr,
> +					   flags_to_prot(unmap_vma->flags),
> +					   to_drm_gem_shmem_obj(op->remap.next->gem.obj)->sgt,
> +					   op->remap.next->gem.offset,
> +					   unmap_start + unmap_range - op->remap.next->va.addr);
> +		if (ret)
> +			return ret;
> +
>  		next_vma = panthor_vm_op_ctx_get_vma(op_ctx);
>  		panthor_vma_init(next_vma, unmap_vma->flags);
>  	}