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

[Usama Arif]

On 22/09/2025 03:14, Lance Yang wrote:
> From: Lance Yang <lance.yang at 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
> 
> Cc: <stable at vger.kernel.org>
> Reported-by: Qun-wei Lin <Qun-wei.Lin at mediatek.com>
> Closes: https://lore.kernel.org/all/a7944523fcc3634607691c35311a5d59d1a3f8d4.camel@mediatek.com
> Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp")
> Suggested-by: David Hildenbrand <david at redhat.com>
> Signed-off-by: Lance Yang <lance.yang at 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 <usamaarif642 at gmail.com>