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

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

Hi Adrian,

On Sun, 19 Oct 2025 04:19:42 +0100
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 | 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) {
> +		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);

This should be part of the previous unmap.

> +		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);

This one too.

> +		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);
>  	}

Overall, it feels more complicated than what I had in mind (see
below, only compile-tested though).

Cheers,

Boris

--->8---
diff --git a/drivers/gpu/drm/panthor/panthor_mmu.c b/drivers/gpu/drm/panthor/panthor_mmu.c
index 6dec4354e378..15718241fd2f 100644
--- a/drivers/gpu/drm/panthor/panthor_mmu.c
+++ b/drivers/gpu/drm/panthor/panthor_mmu.c
@@ -2093,27 +2093,104 @@ static int panthor_gpuva_sm_step_map(struct drm_gpuva_op *op, void *priv)
        return 0;
 }
 
+static void
+align_unmap_range(const struct drm_gpuva_op_remap *op,
+                 u64 *unmap_start, u64 *unmap_end)
+{
+       u64 aligned_unmap_start = ALIGN_DOWN(*unmap_start, SZ_2M);
+       u64 aligned_unmap_end = ALIGN(*unmap_end, SZ_2M);
+
+       /* If we're dealing with a huge page, make sure the unmap region is
+        * aligned on the start of the page.
+        */
+       if (op->prev && aligned_unmap_start < *unmap_start &&
+           op->prev->va.addr <= aligned_unmap_start) {
+               struct panthor_gem_object *bo = to_panthor_bo(op->prev->gem.obj);
+               u64 bo_offset = op->prev->gem.offset + *unmap_start -
+                               op->prev->va.addr;
+               const struct page *pg = bo->base.pages[bo_offset >> PAGE_SHIFT];
+
+               if (folio_size(page_folio(pg)) >= SZ_2M)
+                       *unmap_start = aligned_unmap_start;
+       }
+
+       /* If we're dealing with a huge page, make sure the unmap region is
+        * aligned on the end of the page.
+        */
+       if (op->next && aligned_unmap_end > *unmap_end &&
+           op->next->va.addr + op->next->va.range >= aligned_unmap_end) {
+               struct panthor_gem_object *bo = to_panthor_bo(op->next->gem.obj);
+               u64 bo_offset = op->next->gem.offset + op->next->va.addr +
+                               op->next->va.range - *unmap_end;
+               const struct page *pg = bo->base.pages[bo_offset >> PAGE_SHIFT];
+
+               if (folio_size(page_folio(pg)) >= SZ_2M)
+                       *unmap_end = aligned_unmap_end;
+       }
+}
+
 static int panthor_gpuva_sm_step_remap(struct drm_gpuva_op *op,
                                       void *priv)
 {
        struct panthor_vma *unmap_vma = container_of(op->remap.unmap->va, struct panthor_vma, base);
+       u64 unmap_start, unmap_range, unmap_end, aligned_unmap_start, aligned_unmap_end;
        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;
        int ret;
 
+       /*
+        * 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.
+        */
        drm_gpuva_op_remap_to_unmap_range(&op->remap, &unmap_start, &unmap_range);
-       ret = panthor_vm_unmap_pages(vm, unmap_start, unmap_range);
+       unmap_end = unmap_start + unmap_range;
+       aligned_unmap_start = unmap_start;
+       aligned_unmap_end = unmap_end;
+       align_unmap_range(&op->remap, &aligned_unmap_start, &aligned_unmap_end);
+
+       ret = panthor_vm_unmap_pages(vm, aligned_unmap_start,
+                                    aligned_unmap_end - aligned_unmap_start);
        if (ret)
                return ret;
 
        if (op->remap.prev) {
+               if (aligned_unmap_start < unmap_start) {
+                       struct panthor_gem_object *bo =
+                               to_panthor_bo(op->remap.prev->gem.obj);
+                       u64 bo_offset = op->remap.prev->gem.offset +
+                                       aligned_unmap_start - op->remap.prev->va.addr;
+
+                       ret = panthor_vm_map_pages(vm, aligned_unmap_start,
+                                                  flags_to_prot(unmap_vma->flags),
+                                                  bo->base.sgt, bo_offset,
+                                                  unmap_start - aligned_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) {
+               if (aligned_unmap_end > unmap_end) {
+                       struct panthor_gem_object *bo =
+                               to_panthor_bo(op->remap.next->gem.obj);
+                       u64 bo_offset = op->remap.next->gem.offset + unmap_end -
+                                       op->remap.next->va.addr;
+
+                       ret = panthor_vm_map_pages(vm, unmap_end,
+                                                  flags_to_prot(unmap_vma->flags),
+                                                  bo->base.sgt, bo_offset,
+                                                  aligned_unmap_end - unmap_end);
+                       if (ret)
+                               return ret;
+               }
+
                next_vma = panthor_vm_op_ctx_get_vma(op_ctx);
                panthor_vma_init(next_vma, unmap_vma->flags);
        }