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

[Catalin Marinas <catalin.marinas@arm.com>]

On Tue, Oct 21, 2025 at 12:00:04PM -0700, Yang Shi wrote:
> On Tue, Oct 21, 2025 at 11:14 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(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
> > Cc: <stable@vger.kernel.org> # 6.12.x
> > Cc: Will Deacon <will@kernel.org>
> > Cc: David Hildenbrand <david@redhat.com>
> > Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
> > ---
> >  arch/arm64/mm/copypage.c | 9 +++++++--
> >  1 file changed, 7 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/arm64/mm/copypage.c b/arch/arm64/mm/copypage.c
> > index a86c897017df..40749d0a385c 100644
> > --- a/arch/arm64/mm/copypage.c
> > +++ b/arch/arm64/mm/copypage.c
> > @@ -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));
> 
> I think you should remove the WARN_ON_ONCE in the above hugetlb chunk
> too IIUC. migrate_huge_page_move_mapping() does the similar thing.

Good point. Thanks!

-- 
Catalin