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

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

On 10/21/25 18:39, Adrián Larumbe wrote:
> Hi Akash,
> 
> On 21.10.2025 15:32, Akash Goel wrote:
>>
>>
>> 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'.
> 
> I believe it will always be less than that. 


Thanks Adrian for having a look.

static int panthor_gpuva_sm_step_remap(struct drm_gpuva_op *op,
{
	struct panthor_vma *unmap_vma = container_of(op->remap.unmap->va, 
struct panthor_vma, base);


IIUC, the 'unmap_vma' passed to panthor_gpuva_sm_step_remap() will 
always cover the entire VA range of 'drm_gpuva'.
That's why drm_gpuva_op_remap_to_unmap_range() is called to know the 
exact range to be unmapped.

In __drm_gpuvm_sm_unmap() and __drm_gpuvm_sm_map(), you can see this,

struct drm_gpuva_op_unmap u = { .va = va };


 > What will be equal to unmap_vma->base.va.addr is 
op->remap.prev->va.addr + op->remap.prev->va.range


I think op->remap.prev->va.addr + op->remap.prev->va.range will be equal 
to 'unmap_start' after the call to drm_gpuva_op_remap_to_unmap_range().

Sorry I may have again misunderstood the code.

Please can you check.

Best regards
Akash


>> And for 'op->remap.next', 'op->va.addr' will always be greater than
>> 'unmap_vma->base.va.addr'.
> 
> Yes, I believe so.
> 
>> 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
> 
> Adrian Larumbe