[Linaro-mm-sig] Re: [PATCH 6/8] drm/panthor: Explicit expansion of locked VM region

On Tue, 5 May 2026 16:05:12 +0200 Ketil Johnsen ketil.johnsen@arm.com wrote:

Currently the panthor_vm_lock_region() function will implicitly expand an already locked VM region. This can be problematic because the caller do not reliably know if it needs to call panthor_vm_unlock_region() or not.

Worth noting, there is currently no known issues with this as the code is written today.

This change introduces panthor_vm_expand_region() which will only work if there is already a locked VM region. This again means that the original lock and unlock functions can work as a pair. This pairing is needed for subsequent protected memory changes.

Signed-off-by: Ketil Johnsen ketil.johnsen@arm.com

drivers/gpu/drm/panthor/panthor_mmu.c | 69 +++++++++++++++++++-------- 1 file changed, 50 insertions(+), 19 deletions(-)

diff --git a/drivers/gpu/drm/panthor/panthor_mmu.c b/drivers/gpu/drm/panthor/panthor_mmu.c index fc930ee158a52..07f54176ec1bf 100644 --- a/drivers/gpu/drm/panthor/panthor_mmu.c +++ b/drivers/gpu/drm/panthor/panthor_mmu.c @@ -1701,15 +1701,36 @@ static int panthor_vm_lock_region(struct panthor_vm *vm, u64 start, u64 size) struct panthor_device *ptdev = vm->ptdev; int ret = 0;

• /* sm_step_remap() can call panthor_vm_lock_region() to account for

• * the wider unmap needed when doing a partial huge page unamp. We
  

• * need to ignore the lock if it's already part of the locked region.
  

• */
  

• if (start >= vm->locked_region.start &&

•    start + size <= vm->locked_region.start + vm->locked_region.size)
  

• return 0;
  

• if (drm_WARN_ON(&ptdev->base, vm->locked_region.size))

• return -EINVAL;
  

How about we have a helper called panthor_vm_apply_as_lock() that would only take care of the AS_LOCKADDR() sequence. panthor_vm_lock_region() would have this WARN_ON(), the pack_region_range() and a call to panthor_vm_apply_as_lock(). Similarly, panthor_vm_expand_locked_region() would rely on panthor_vm_apply_as_lock() to apply the expanded lock.

• mutex_lock(&ptdev->mmu->as.slots_lock);
• if (vm->as.id >= 0 && size) {

• /* Lock the region that needs to be updated */
  

• gpu_write64(ptdev, AS_LOCKADDR(vm->as.id),
  

• 	    pack_region_range(ptdev, &start, &size));
  

• /* If the lock succeeded, update the locked_region info. */
  

• ret = as_send_cmd_and_wait(ptdev, vm->as.id, AS_COMMAND_LOCK);
  

• }

• /* sm_step_remap() may need a locked region that isn't a strict superset

• if (!ret) {

• vm->locked_region.start = start;
  

• vm->locked_region.size = size;
  

• }
• mutex_unlock(&ptdev->mmu->as.slots_lock);
• return ret;

+}

+static int panthor_vm_expand_region(struct panthor_vm *vm, u64 start, u64 size)

s/panthor_vm_expand_region/panthor_vm_expand_locked_region/

+{

• struct panthor_device *ptdev = vm->ptdev;
• u64 end;
• int ret = 0;
• /* This function is here to handle the following case:

• * sm_step_remap() may need a locked region that isn't a strict superset
  

  • of the original one because of having to extend unmap boundaries beyond
  • it to deal with partial unmaps of transparent huge pages. What we want
  • in those cases is to lock the union of both regions. The new region must

@@ -1717,16 +1738,24 @@ static int panthor_vm_lock_region(struct panthor_vm *vm, u64 start, u64 size) * boundaries in a remap operation can only shift up or down respectively, * but never otherwise. */

• if (vm->locked_region.size) {

• u64 end = max(vm->locked_region.start + vm->locked_region.size,
  

• 	      start + size);
  

• drm_WARN_ON_ONCE(&vm->ptdev->base, (start + size <= vm->locked_region.start) ||
  

• 		 (start >= vm->locked_region.start + vm->locked_region.size));
  

• /* This function can only expand an already locked region */
• if (drm_WARN_ON(&ptdev->base, !vm->locked_region.size))

• return -EINVAL;
  

• start = min(start, vm->locked_region.start);
  

• size = end - start;
  

• }

• /* Early out if requested range is already locked */
• if (start >= vm->locked_region.start &&

•    start + size <= vm->locked_region.start + vm->locked_region.size)
  

• return 0;
  

• end = max(vm->locked_region.start + vm->locked_region.size,

•   start + size);
  

• drm_WARN_ON_ONCE(&ptdev->base, (start + size <= vm->locked_region.start) ||

• 	 (start >= vm->locked_region.start + vm->locked_region.size));
  

• start = min(start, vm->locked_region.start);
• size = end - start;

mutex_lock(&ptdev->mmu->as.slots_lock); if (vm->as.id >= 0 && size) { @@ -2252,11 +2281,13 @@ static int panthor_gpuva_sm_step_remap(struct drm_gpuva_op *op, unmap_hugepage_align(&op->remap, &unmap_start, &unmap_range); /* If the range changed, we might have to lock a wider region to guarantee

• * atomicity. panthor_vm_lock_region() bails out early if the new region
  

• * is already part of the locked region, so no need to do this check here.
  

• * atomicity.
  

  */ if (!unmap_vma->evicted) {

• panthor_vm_lock_region(vm, unmap_start, unmap_range);
  

• ret = panthor_vm_expand_region(vm, unmap_start, unmap_range);
  

• if (ret)
  

• 	return ret;
  

• panthor_vm_unmap_pages(vm, unmap_start, unmap_range); }