[PATCH] arm64: mte: Fix copy_highpage() warning on hugetlb folios

[Catalin Marinas]

Commit 25c17c4b55de ("hugetlb: arm64: add mte support") improved the
copy_highpage() function to update the tags in the destination hugetlb
folio. However, when the source folio isn't tagged, the code takes the
non-hugetlb path where try_page_mte_tagging() warns as the destination
is a hugetlb folio:

  WARNING: CPU: 0 PID: 363 at arch/arm64/include/asm/mte.h:58 copy_highpage+0x1d4/0x2d8
  [...]
  pc : copy_highpage+0x1d4/0x2d8
  lr : copy_highpage+0x78/0x2d8
  [...]
  Call trace:
   copy_highpage+0x1d4/0x2d8 (P)
   copy_highpage+0x78/0x2d8 (L)
   copy_user_highpage+0x20/0x48
   copy_user_large_folio+0x1bc/0x268
   hugetlb_wp+0x190/0x860
   hugetlb_fault+0xa28/0xc10
   handle_mm_fault+0x2a0/0x2c0
   do_page_fault+0x12c/0x578
   do_mem_abort+0x4c/0xa8
   el0_da+0x44/0xb0
   el0t_64_sync_handler+0xc4/0x138
   el0t_64_sync+0x198/0x1a0

Change the check for the tagged status of the source folio so that it
does not fall through the non-hugetlb case. In addition, only perform
the copy (for the full folio) if the source page is the folio head and
warn if the destination folio is already tagged, for symmetry with the
non-hugetlb case.

Fixes: 25c17c4b55de ("hugetlb: arm64: add mte support")
Signed-off-by: Catalin Marinas <catalin.marinas at arm.com>
Reported-by: Sasha Levin <sashal at kernel.org>
Cc: Yang Shi <yang at os.amperecomputing.com>
Cc: David Hildenbrand <david at redhat.com>
Cc: Will Deacon <will at kernel.org>
Link: https://lore.kernel.org/r/Z0STR6VLt2MCalnY@sashalap
---
 arch/arm64/mm/copypage.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/mm/copypage.c b/arch/arm64/mm/copypage.c
index 87b3f1a25535..a86c897017df 100644
--- a/arch/arm64/mm/copypage.c
+++ b/arch/arm64/mm/copypage.c
@@ -30,11 +30,13 @@ void copy_highpage(struct page *to, struct page *from)
 	if (!system_supports_mte())
 		return;
 
-	if (folio_test_hugetlb(src) &&
-	    folio_test_hugetlb_mte_tagged(src)) {
-		if (!folio_try_hugetlb_mte_tagging(dst))
+	if (folio_test_hugetlb(src)) {
+		if (!folio_test_hugetlb_mte_tagged(src) ||
+		    from != folio_page(src, 0))
 			return;
 
+		WARN_ON_ONCE(!folio_try_hugetlb_mte_tagging(dst));
+
 		/*
 		 * Populate tags for all subpages.
 		 *