Re: [PATCH] drm/xe/userptr: properly setup pfn_flags_mask

["Hellstrom, Thomas" <thomas.hellstrom@intel.com>]

On Wed, 2025-02-26 at 17:17 +0000, Matthew Auld wrote:
> Currently we just leave it uninitialised, which at first looks
> harmless,
> however we also don't zero out the pfn array, and with pfn_flags_mask
> the idea is to be able set individual flags for a given range of pfn
> or
> completely ignore them, outside of default_flags. So here we end up
> with
> pfn[i] & pfn_flags_mask, and if both are uninitialised we might get
> back
> an unexpected flags value, like asking for read only with
> default_flags,
> but getting back write on top, leading to potentially bogus
> behaviour.
> 
> To fix this ensure we zero the pfn_flags_mask, such that hmm only
> considers the default_flags and not also the initial pfn[i] value.
> 
> Fixes: 81e058a3e7fd ("drm/xe: Introduce helper to populate userptr")
> Signed-off-by: Matthew Auld <matthew.auld@intel.com>
> Cc: Matthew Brost <matthew.brost@intel.com>
> Cc: Thomas Hellström <thomas.hellstrom@intel.com>
> Cc: <stable@vger.kernel.org> # v6.10+
> ---
>  drivers/gpu/drm/xe/xe_hmm.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/gpu/drm/xe/xe_hmm.c
> b/drivers/gpu/drm/xe/xe_hmm.c
> index 089834467880..8c3cd65fa4b3 100644
> --- a/drivers/gpu/drm/xe/xe_hmm.c
> +++ b/drivers/gpu/drm/xe/xe_hmm.c
> @@ -206,6 +206,7 @@ int xe_hmm_userptr_populate_range(struct
> xe_userptr_vma *uvma,
>  		goto free_pfns;
>  	}
>  
> +	hmm_range.pfn_flags_mask = 0;
>  	hmm_range.default_flags = flags;
>  	hmm_range.hmm_pfns = pfns;
>  	hmm_range.notifier = &userptr->notifier;

Is there a chance we can modify the function to have an initializer for
hmm_range

struct hmm_range hmm_range = {...

That'd also make adding fields to struct hmm_range less fragile.

Either way,
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>