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

[Kefeng Wang]

On 2025/10/22 3:00, Yang Shi wrote:
> On Tue, Oct 21, 2025 at 11:14 AM Catalin Marinas
> <catalin.marinas at 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

no "__" prefix

>> 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 at arm.com>
>> Reported-by: syzbot+d1974fc28545a3e6218b at syzkaller.appspotmail.com
>> Link: https://lore.kernel.org/r/68dda1ae.a00a0220.102ee.0065.GAE@google.com
>> Cc: <stable at vger.kernel.org> # 6.12.x
>> Cc: Will Deacon <will at kernel.org>
>> Cc: David Hildenbrand <david at redhat.com>
>> Cc: Kefeng Wang <wangkefeng.wang at 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.
> 

Yes, it's better to remove WARN_ON_ONCE for hugetlb too.

> Thanks,
> Yang
> 
>> +               /*
>> +                * 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);
>>