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

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 0609139 ("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.

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
Reviewed-by: Yang Shi <yang@os.amperecomputing.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: ctmarinas

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);