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[2003:d8:2f2a:e200:f98f:8d71:83f:f88]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-461383b7b9csm253669505e9.2.2025.09.22.10.59.01 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 22 Sep 2025 10:59:02 -0700 (PDT) Message-ID: Date: Mon, 22 Sep 2025 19:59:00 +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 To: Catalin Marinas , Lance Yang Cc: akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, usamaarif642@gmail.com, yuzhao@google.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, baohua@kernel.org, voidice@gmail.com, Liam.Howlett@oracle.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: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 22.09.25 19:24, Catalin Marinas wrote: > On Mon, Sep 22, 2025 at 10:14:58AM +0800, 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 > > Functionally, the patch looks fine, both with and without MTE. > > Reviewed-by: Catalin Marinas > >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >> index 32e0ec2dde36..28d4b02a1aa5 100644 >> --- a/mm/huge_memory.c >> +++ b/mm/huge_memory.c >> @@ -4104,29 +4104,20 @@ static unsigned long deferred_split_count(struct shrinker *shrink, >> static bool thp_underused(struct folio *folio) >> { >> int num_zero_pages = 0, num_filled_pages = 0; >> - void *kaddr; >> int i; >> >> for (i = 0; i < folio_nr_pages(folio); i++) { >> - kaddr = kmap_local_folio(folio, i * PAGE_SIZE); >> - if (!memchr_inv(kaddr, 0, PAGE_SIZE)) { >> - num_zero_pages++; >> - if (num_zero_pages > khugepaged_max_ptes_none) { >> - kunmap_local(kaddr); >> + if (pages_identical(folio_page(folio, i), ZERO_PAGE(0))) { >> + if (++num_zero_pages > khugepaged_max_ptes_none) >> return true; > > I wonder what the overhead of doing a memcmp() vs memchr_inv() is. The > former will need to read from two places. If it's noticeable, it would > affect architectures that don't have an MTE equivalent. > > Alternatively we could introduce something like folio_has_metadata() > which on arm64 simply checks PG_mte_tagged. We discussed something similar in the other thread (I suggested page_is_mergable()). I'd prefer to use pages_identical() for now, so we have the same logic here and in ksm code. (this patch here almost looks like a cleanup :) ) If this becomes a problem, what we could do is in pages_identical() would be simply doing the memchr_inv() in case is_zero_pfn(). KSM might benefit from that as well when merging with the shared zeropage through try_to_merge_with_zero_page(). -- Cheers David / dhildenb