Subject: Re: [PATCH 6/8] drm/panthor: Explicit expansion of locked VM region

From: Boris Brezillon <[email-masked]>
Date: Tue, 5 May 2026 18:32:44 +0200

On Tue,  5 May 2026 16:05:12 +0200
Ketil Johnsen <[email-masked]> 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 <[email-masked]>
> ---
>  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);
>  	}
>