Re: [PATCH v2 1/2] drm/panthor: Extend VM locked region for remap case to be a superset

[Liviu Dudau <liviu.dudau@arm.com>]

On Wed, Apr 08, 2026 at 08:12:23PM +0100, Adrián Larumbe wrote:
> In the event of an sm_step_remap() that leads to a partial unmap of a
> transparent huge page, the new locked region required by an extended unmap
> might not be a superset of the original one. Then, if it leaves a portion
> of the initially requested one out, the ensuing map will trigger a warning.
> 
> Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
> Fixes: 8e7460eac786 ("drm/panthor: Support partial unmaps of huge pages")
> Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>

Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>

Best regards,
Liviu

> ---
>  drivers/gpu/drm/panthor/panthor_mmu.c | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_mmu.c b/drivers/gpu/drm/panthor/panthor_mmu.c
> index fa8b31df85c9..bd41c892beb7 100644
> --- a/drivers/gpu/drm/panthor/panthor_mmu.c
> +++ b/drivers/gpu/drm/panthor/panthor_mmu.c
> @@ -1709,6 +1709,25 @@ static int panthor_vm_lock_region(struct panthor_vm *vm, u64 start, u64 size)
>  	    start + size <= vm->locked_region.start + vm->locked_region.size)
>  		return 0;
>  
> +	/* 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
> +	 * always overlap with the original one, because the upper and lower unmap
> +	 * 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));
> +
> +		start = min(start, vm->locked_region.start);
> +		size = end - start;
> +	}
> +
>  	mutex_lock(&ptdev->mmu->as.slots_lock);
>  	if (vm->as.id >= 0 && size) {
>  		/* Lock the region that needs to be updated */
> -- 
> 2.53.0
> 

-- 
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