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[2003:d8:2f4f:700:c9db:579f:8b2b:717c]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3fcb01a9049sm9757269f8f.61.2025.09.23.09.40.07 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 23 Sep 2025 09:40:09 -0700 (PDT) Message-ID: <45a7aa69-16df-4809-8ccd-c08c404d55cb@redhat.com> Date: Tue, 23 Sep 2025 18:40:06 +0200 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 Cc: Lance Yang , 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 Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250923_094014_781732_CAC6F23D X-CRM114-Status: GOOD ( 23.53 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 23.09.25 18:14, Catalin Marinas wrote: > On Tue, Sep 23, 2025 at 12:52:06PM +0100, Catalin Marinas wrote: >> I just realised that on arm64 with MTE we won't get any merging with the >> zero page even if the user page isn't mapped with PROT_MTE. In >> cpu_enable_mte() we zero the tags in the zero page and set >> PG_mte_tagged. The reason is that we want to use the zero page with >> PROT_MTE mappings (until tag setting causes CoW). Hmm, the arm64 >> memcmp_pages() messed up KSM merging with the zero page even before this >> patch. > [...] >> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c >> index e5e773844889..72a1dfc54659 100644 >> --- a/arch/arm64/kernel/mte.c >> +++ b/arch/arm64/kernel/mte.c >> @@ -73,6 +73,8 @@ int memcmp_pages(struct page *page1, struct page *page2) >> { >> char *addr1, *addr2; >> int ret; >> + bool page1_tagged = page_mte_tagged(page1) && !is_zero_page(page1); >> + bool page2_tagged = page_mte_tagged(page2) && !is_zero_page(page2); >> >> addr1 = page_address(page1); >> addr2 = page_address(page2); >> @@ -83,11 +85,10 @@ int memcmp_pages(struct page *page1, struct page *page2) >> >> /* >> * If the page content is identical but at least one of the pages is >> - * tagged, return non-zero to avoid KSM merging. If only one of the >> - * pages is tagged, __set_ptes() may zero or change the tags of the >> - * other page via mte_sync_tags(). >> + * tagged, return non-zero to avoid KSM merging. Ignore the zero page >> + * since it is always tagged with the tags cleared. >> */ >> - if (page_mte_tagged(page1) || page_mte_tagged(page2)) >> + if (page1_tagged || page2_tagged) >> return addr1 != addr2; >> >> return ret; > > Unrelated to this discussion, I got an internal report that Linux hangs > during boot with CONFIG_DEFERRED_STRUCT_PAGE_INIT because > try_page_mte_tagging() locks up on uninitialised page flags. > > Since we (always?) map the zero page as pte_special(), set_pte_at() Yes. (if pte_special is implemented by the arch of course) > won't check if the tags have to be initialised, so we can skip the > PG_mte_tagged altogether. We actually had this code for some time until > we introduced the pte_special() check in set_pte_at(). > > So alternative patch that also fixes the deferred struct page init (on > the assumptions that the zero page is always mapped as pte_special(): > LGTM! -- Cheers David / dhildenb