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

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Message-ID: <20250923021043.nx2k4ftzle6e2jht@master>
Date: Tue, 23 Sep 2025 02:10:43 +0000
From: Wei Yang <richard.weiyang@...il.com>
To: Lance Yang <lance.yang@...ux.dev>
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Subject: Re: [PATCH 1/1] mm/thp: fix MTE tag mismatch when replacing
 zero-filled subpages

On Mon, Sep 22, 2025 at 10:14:58AM +0800, Lance Yang wrote:
>From: Lance Yang <lance.yang@...ux.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@...r.kernel.org>
>Reported-by: Qun-wei Lin <Qun-wei.Lin@...iatek.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@...hat.com>
>Signed-off-by: Lance Yang <lance.yang@...ux.dev>

Nice catch.

Reviewed-by: Wei Yang <richard.weiyang@...il.com>

-- 
Wei Yang
Help you, Help me