Re: [PATCH 1/1] mm/thp: fix MTE tag mismatch when replacing zero-filled subpages

On 22/09/2025 03:14, Lance Yang wrote:

From: Lance Yang <lance.yang@linux.dev>

When both THP and MTE are enabled, splitting a THP and replacing its
zero-filled subpages with the shared zeropage can cause MTE tag mismatch
faults in userspace.

Remapping zero-filled subpages to the shared zeropage is unsafe, as the
zeropage has a fixed tag of zero, which may not match the tag expected by
the userspace pointer.

KSM already avoids this problem by using memcmp_pages(), which on arm64
intentionally reports MTE-tagged pages as non-identical to prevent unsafe
merging.

As suggested by David[1], this patch adopts the same pattern, replacing the
memchr_inv() byte-level check with a call to pages_identical(). This
leverages existing architecture-specific logic to determine if a page is
truly identical to the shared zeropage.

Having both the THP shrinker and KSM rely on pages_identical() makes the
design more future-proof, IMO. Instead of handling quirks in generic code,
we just let the architecture decide what makes two pages identical.

[1] https://lore.kernel.org/all/ca2106a3-4bb2-4457-81af-301fd99fbef4@redhat.com (local)

Cc: <redacted>
Reported-by: Qun-wei Lin <redacted>
Closes: https://lore.kernel.org/all/a7944523fcc3634607691c35311a5d59d1a3f8d4.camel@mediatek.com (local)
Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp")
Suggested-by: David Hildenbrand <redacted>
Signed-off-by: Lance Yang <lance.yang@linux.dev>
---
Tested on x86_64 and on QEMU for arm64 (with and without MTE support),
and the fix works as expected.

 mm/huge_memory.c | 15 +++------------
 mm/migrate.c     |  8 +-------
 2 files changed, 4 insertions(+), 19 deletions(-)

Thanks for the fix!

Acked-by: Usama Arif <redacted>