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Mon, 22 Sep 2025 01:24:05 -0700 (PDT) Message-ID: <8884d7df-9709-4f5f-abfc-6b74aa220295@gmail.com> Date: Mon, 22 Sep 2025 10:24:01 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/1] mm/thp: fix MTE tag mismatch when replacing zero-filled subpages Content-Language: en-GB To: Lance Yang , akpm@linux-foundation.org, david@redhat.com, lorenzo.stoakes@oracle.com Cc: yuzhao@google.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, baohua@kernel.org, voidice@gmail.com, Liam.Howlett@oracle.com, catalin.marinas@arm.com, cerasuolodomenico@gmail.com, hannes@cmpxchg.org, kaleshsingh@google.com, npache@redhat.com, riel@surriel.com, roman.gushchin@linux.dev, rppt@kernel.org, ryan.roberts@arm.com, dev.jain@arm.com, ryncsn@gmail.com, shakeel.butt@linux.dev, surenb@google.com, hughd@google.com, willy@infradead.org, matthew.brost@intel.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, qun-wei.lin@mediatek.com, Andrew.Yang@mediatek.com, casper.li@mediatek.com, chinwen.chang@mediatek.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mediatek@lists.infradead.org, linux-mm@kvack.org, ioworker0@gmail.com, stable@vger.kernel.org References: <20250922021458.68123-1-lance.yang@linux.dev> From: Usama Arif In-Reply-To: <20250922021458.68123-1-lance.yang@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 22/09/2025 03:14, Lance Yang wrote: > From: Lance Yang > > 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: > Reported-by: Qun-wei Lin > 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 > Signed-off-by: Lance Yang > --- > 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