Re: [PATCH 1/3] KVM: arm64: Clear the whole hugetlb folio when sanitising MTE tags

[Fuad Tabba]

Hi Lorenzo,

On Tue, 20 Oct 2026 12:52:54 +0100, Lorenzo Stoakes (ARM)
<ljs@xxxxxxxxxx> wrote:
[...]
> On Mon, Oct 19, 2026 at 02:21:01PM +0100, Fuad Tabba wrote:
> > sanitise_mte_tags() clears the tags of the pages about to be mapped at
> > stage-2 before marking them tagged. For a hugetlb folio the flag covers
> > every page in the folio, but the loop only covers the mapping being
> > installed. Stage-2 maps a folio in pieces for the contiguous hugetlb
> > sizes, and for any size under dirty logging. The first fault then marks
> > the whole folio after clearing only its own piece, and the rest keeps
> > its previous owner's tags for the guest to read.
>
> I'm guessing the folio size is always >= size here?

Yes, the stage-2 mapping of a hugetlb VMA is never larger than its
hugetlb page size, so it always sits within one folio.

[...]
> > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
[...]
> > @@ -1514,7 +1514,8 @@ static void sanitise_mte_tags(struct kvm *kvm, kvm_pfn_t pfn,
> >          if (folio_test_hugetlb(folio)) {
> >                  /* Hugetlb has MTE flags set on head page only */
>
> Is this comment redundant now?

I'd keep it: it's why the whole folio has to be cleared, as the flags
on the head page cover every page in it.

> >                  if (folio_try_hugetlb_mte_tagging(folio)) {
> > -                        for (i = 0; i < nr_pages; i++, page++)
> > +                        page = folio_page(folio, 0);
> > +                        for (i = 0; i < folio_nr_pages(folio); i++, page++)
>
> I know it's a stable fix, but the fact that this pattern exists in multiple
> places really argues for a mte_clear_folio_tags(), which would make this
> trivially:
>
>         mte_clear_folio_tags(folio);
>
> And avoid the re-use of page etc.
>
> It's not a huge change and makes the next patch in the series simpler too so
> arguably could be part of the series?

I think you're right about the helper, but I'd rather keep the fixes
as small as possible for the backports. I'll send
mte_clear_folio_tags() as a separate patch on top of these, using it
in mte_sync_tags() too. In v2 the loop uses folio_page(folio, i)
rather than reusing page.

Thanks,
/fuad