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

[Akash Goel <akash.goel@arm.com>]

On 10/19/25 04:19, Adrián Larumbe wrote:
> Commit 33729a5fc0ca ("iommu/io-pgtable-arm: Remove split on unmap
> behavior") did away with the treatment of partial unmaps of huge IOPTEs.
> 

Sorry have a doubt.

Corresponding to the commit 33729a5fc0ca, can we now remove the code to 
pre-allocate L3 page table pages i.e. 'op_ctx->rsvd_page_tables.pages' 
inside panthor_vm_prepare_unmap_op_ctx() ?.

> 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 | 129 +++++++++++++++++++++++++-
>   1 file changed, 126 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_mmu.c b/drivers/gpu/drm/panthor/panthor_mmu.c
> index 2d041a2e75e9..f9d200e57c04 100644
> --- a/drivers/gpu/drm/panthor/panthor_mmu.c
> +++ b/drivers/gpu/drm/panthor/panthor_mmu.c
> @@ -2093,6 +2093,98 @@ static int panthor_gpuva_sm_step_map(struct drm_gpuva_op *op, void *priv)
>   	return 0;
>   }
>   
> +static bool
> +is_huge_page_partial_unmap(const struct panthor_vma *unmap_vma,
> +			   const struct drm_gpuva_op_map *op,
> +			   u64 unmap_start, u64 unmap_range,
> +			   u64 sz2m_prev, u64 sz2m_next)
> +{
> +	size_t pgcount, pgsize;
> +	const struct page *pg;
> +	pgoff_t bo_offset;
> +
> +	if (op->va.addr < unmap_vma->base.va.addr) {


Sorry, another doubt.

Will this condition ever be true ?

For 'op->remap.prev', 'op->va.addr' will always be equal to 
'unmap_vma->base.va.addr'.
And for 'op->remap.next', 'op->va.addr' will always be greater than 
'unmap_vma->base.va.addr'.

Please can you clarify.

Best regards
Akash


> +		bo_offset = unmap_start - unmap_vma->base.va.addr + unmap_vma->base.gem.offset;
> +		sz2m_prev = ALIGN_DOWN(unmap_start, SZ_2M);
> +		sz2m_next = ALIGN(unmap_start + 1, SZ_2M);
> +		pgsize = get_pgsize(unmap_start, unmap_range, &pgcount);
> +
> +	} else {
> +		bo_offset = ((unmap_start + unmap_range - 1) - unmap_vma->base.va.addr)
> +			+ unmap_vma->base.gem.offset;
> +		sz2m_prev = ALIGN_DOWN(unmap_start + unmap_range - 1, SZ_2M);
> +		sz2m_next = ALIGN(unmap_start + unmap_range, SZ_2M);
> +		pgsize = get_pgsize(sz2m_prev, unmap_start + unmap_range - sz2m_prev, &pgcount);
> +	}
> +
> +	pg = to_panthor_bo(unmap_vma->base.gem.obj)->base.pages[bo_offset >> PAGE_SHIFT];
> +
> +	if (pgsize == SZ_4K && folio_order(page_folio(pg)) == PMD_ORDER &&
> +	    unmap_vma->base.va.addr <= sz2m_prev && unmap_vma->base.va.addr +
> +	    unmap_vma->base.va.range >= sz2m_next)
> +		return true;
> +
> +	return false;
> +}
> +
> +struct remap_params {
> +	u64 prev_unmap_start, prev_unmap_range;
> +	u64 prev_remap_start, prev_remap_range;
> +	u64 next_unmap_start, next_unmap_range;
> +	u64 next_remap_start, next_remap_range;
> +	u64 unmap_start, unmap_range;
> +};
> +
> +static struct remap_params
> +get_map_unmap_intervals(const struct drm_gpuva_op_remap *op,
> +			const struct panthor_vma *unmap_vma)
> +{
> +	u64 unmap_start, unmap_range, sz2m_prev, sz2m_next;
> +	struct remap_params params = {0};
> +
> +	drm_gpuva_op_remap_to_unmap_range(op, &unmap_start, &unmap_range);
> +
> +	if (op->prev) {
> +		sz2m_prev = ALIGN_DOWN(unmap_start, SZ_2M);
> +		sz2m_next = ALIGN(unmap_start + 1, SZ_2M);
> +
> +		if (is_huge_page_partial_unmap(unmap_vma, op->prev, unmap_start,
> +					       unmap_range, sz2m_prev, sz2m_next)) {
> +			params.prev_unmap_start = sz2m_prev;
> +			params.prev_unmap_range = SZ_2M;
> +			params.prev_remap_start = sz2m_prev;
> +			params.prev_remap_range = unmap_start & (SZ_2M - 1);
> +
> +			u64 diff = min(sz2m_next - unmap_start, unmap_range);
> +
> +			unmap_range -= diff;
> +			unmap_start += diff;
> +		}
> +	}
> +
> +	if (op->next) {
> +		sz2m_prev = ALIGN_DOWN(unmap_start + unmap_range - 1, SZ_2M);
> +		sz2m_next = ALIGN(unmap_start + unmap_range, SZ_2M);
> +
> +		if (is_huge_page_partial_unmap(unmap_vma, op->next, unmap_start,
> +					       unmap_range, sz2m_prev, sz2m_next)) {
> +			if (unmap_range) {
> +				params.next_unmap_start = sz2m_prev;
> +				params.next_unmap_range = SZ_2M;
> +				unmap_range -= op->next->va.addr & (SZ_2M - 1);
> +			}
> +
> +			params.next_remap_start = op->next->va.addr;
> +			params.next_remap_range = SZ_2M - (op->next->va.addr & (SZ_2M - 1));
> +		}
> +	}
> +
> +	params.unmap_start = unmap_start;
> +	params.unmap_range = unmap_range;
> +
> +	return params;
> +}
> +
>   static int panthor_gpuva_sm_step_remap(struct drm_gpuva_op *op,
>   				       void *priv)
>   {
> @@ -2100,20 +2192,51 @@ static int panthor_gpuva_sm_step_remap(struct drm_gpuva_op *op,
>   	struct panthor_vm *vm = priv;
>   	struct panthor_vm_op_ctx *op_ctx = vm->op_ctx;
>   	struct panthor_vma *prev_vma = NULL, *next_vma = NULL;
> -	u64 unmap_start, unmap_range;
> +	struct remap_params params;
>   	int ret;
>   
> -	drm_gpuva_op_remap_to_unmap_range(&op->remap, &unmap_start, &unmap_range);
> -	ret = panthor_vm_unmap_pages(vm, 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.
> +	 */
> +	params = get_map_unmap_intervals(&op->remap, unmap_vma);
> +
> +	ret = panthor_vm_unmap_pages(vm, params.unmap_start, params.unmap_range);
>   	if (ret)
>   		return ret;
>   
>   	if (op->remap.prev) {
> +		ret = panthor_vm_unmap_pages(vm, params.prev_unmap_start,
> +					     params.prev_unmap_range);
> +		if (ret)
> +			return ret;
> +		ret = panthor_vm_map_pages(vm, params.prev_remap_start,
> +					   flags_to_prot(unmap_vma->flags),
> +					   to_drm_gem_shmem_obj(op->remap.prev->gem.obj)->sgt,
> +					   op->remap.prev->gem.offset, params.prev_remap_range);
> +		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_unmap_pages(vm, params.next_unmap_start,
> +					     params.next_unmap_range);
> +		if (ret)
> +			return ret;
> +
> +		ret = panthor_vm_map_pages(vm, params.next_remap_start,
> +					   flags_to_prot(unmap_vma->flags),
> +					   to_drm_gem_shmem_obj(op->remap.next->gem.obj)->sgt,
> +					   op->remap.next->gem.offset, params.next_remap_range);
> +		if (ret)
> +			return ret;
> +
>   		next_vma = panthor_vm_op_ctx_get_vma(op_ctx);
>   		panthor_vma_init(next_vma, unmap_vma->flags);
>   	}
> 
> base-commit: 7fb19ea1ec6aa85c75905b1fd732d50801e7fb28
> prerequisite-patch-id: 3b0f61bfc22a616a205ff7c15d546d2049fd53de