Re: [PATCH v2] arm64: mte: Do not warn if the page is already tagged in copy_highpage()

[Yang Shi <shy828301@gmail.com>]

On Wed, Oct 22, 2025 at 3:20 AM Catalin Marinas <catalin.marinas@arm.com> wrote:
>
> The arm64 copy_highpage() assumes that the destination page is newly
> allocated and not MTE-tagged (PG_mte_tagged unset) and warns
> accordingly. However, following commit 060913999d7a ("mm: migrate:
> support poisoned recover from migrate folio"), folio_mc_copy() is called
> before __folio_migrate_mapping(). If the latter fails (-EAGAIN), the
> copy will be done again to the same destination page. Since
> copy_highpage() already set the PG_mte_tagged flag, this second copy
> will warn.
>
> Replace the WARN_ON_ONCE(page already tagged) in the arm64
> copy_highpage() with a comment.
>
> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> Reported-by: syzbot+d1974fc28545a3e6218b@syzkaller.appspotmail.com
> Link: https://lore.kernel.org/r/68dda1ae.a00a0220.102ee.0065.GAE@google.com
> Reviewed-by: David Hildenbrand <david@redhat.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
> Cc: <stable@vger.kernel.org> # 6.12.x
> ---
>
> v2:
> - Also remove the WARN_ON_ONCE for hugetlb pages
> - Drop a double-underscore in the commit log

Reviewed-by: Yang Shi <yang@os.amperecomputing.com>

>
>  arch/arm64/mm/copypage.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/mm/copypage.c b/arch/arm64/mm/copypage.c
> index a86c897017df..cd5912ba617b 100644
> --- a/arch/arm64/mm/copypage.c
> +++ b/arch/arm64/mm/copypage.c
> @@ -35,7 +35,7 @@ void copy_highpage(struct page *to, struct page *from)
>                     from != folio_page(src, 0))
>                         return;
>
> -               WARN_ON_ONCE(!folio_try_hugetlb_mte_tagging(dst));
> +               folio_try_hugetlb_mte_tagging(dst);
>
>                 /*
>                  * Populate tags for all subpages.
> @@ -51,8 +51,13 @@ void copy_highpage(struct page *to, struct page *from)
>                 }
>                 folio_set_hugetlb_mte_tagged(dst);
>         } else if (page_mte_tagged(from)) {
> -               /* It's a new page, shouldn't have been tagged yet */
> -               WARN_ON_ONCE(!try_page_mte_tagging(to));
> +               /*
> +                * Most of the time it's a new page that shouldn't have been
> +                * tagged yet. However, folio migration can end up reusing the
> +                * same page without untagging it. Ignore the warning if the
> +                * page is already tagged.
> +                */
> +               try_page_mte_tagging(to);
>
>                 mte_copy_page_tags(kto, kfrom);
>                 set_page_mte_tagged(to);
>