Re: [PATCH 2/3] KVM: arm64: Clear the whole hugetlb folio when storing MTE tags

[Lorenzo Stoakes (ARM)]

On Mon, Oct 19, 2026 at 02:21:02PM +0100, Fuad Tabba wrote:
> When KVM_ARM_MTE_COPY_TAGS copies tags into a page, it marks the page as
> tagged. For a hugetlb folio the flag covers every page in the folio. If
> the folio wasn't tagged yet, its other pages are marked as tagged with
> their previous owner's tags still in place. The stage-2 fault path then
> maps them into the guest without clearing them. The ioctl's read side
> also copies them out to the VMM.
>
> Clear the whole folio on its first tagging, before copying in this
> page's tags.
>
> Fixes: 25c17c4b55def ("hugetlb: arm64: add mte support")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Fuad Tabba <fuad.tabba@xxxxxxxxx>

Logic LGTM, with some nits below:

Reviewed-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>

> ---
>  arch/arm64/kvm/guest.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> index b01d6622b8720..cda655fbff657 100644
> --- a/arch/arm64/kvm/guest.c
> +++ b/arch/arm64/kvm/guest.c
> @@ -1012,6 +1012,7 @@ int kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
>  		void *maddr;
>  		unsigned long num_tags;
>  		struct folio *folio;
> +		unsigned long i;
>
>  		if (!page) {
>  			ret = -EFAULT;
> @@ -1042,12 +1043,14 @@ int kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
>  			/*
>  			 * Only locking to serialise with a concurrent
>  			 * __set_ptes() in the VMM but still overriding the
> -			 * tags, hence ignoring the return value.
> +			 * tags. A hugetlb folio is marked tagged as a whole,
> +			 * so its first tagging clears every page.
>  			 */
> -			if (folio_test_hugetlb(folio))
> -				folio_try_hugetlb_mte_tagging(folio);
> -			else
> +			if (!folio_test_hugetlb(folio))
>  				try_page_mte_tagging(page);
> +			else if (folio_try_hugetlb_mte_tagging(folio))

Slightly awkward logic, testing a negative then the negative of the negative
(i.e. else) plus folio_try_hugetlb_mte_tagging(), makes it a little harder to
understand.

With mte_clear_folio_tags() you can make life simpler:

	if (folio_test_hugetlb(folio)) {
		if (folio_try_hugetlb_mte_tagging(folio))
			mte_clear_folio_tags(folio);
	} else {
		try_page_mte_tagging(page);
	}

> +				for (i = 0; i < folio_nr_pages(folio); i++)
> +					mte_clear_page_tags(page_address(folio_page(folio, i)));

As above, similar comment to 1/3 re: mte_clear_folio_tags(). Would be less churn also.

>  			num_tags = mte_copy_tags_from_user(maddr, tags,
>  							MTE_GRANULES_PER_PAGE);
>
> --
> 2.47.3
>
>

--
Cheers, Lorenzo