From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793557139; b=ait0dOPQ/Fgq9DOe8Mznny3MZLA2o6UOenm+Ui3L0b/tOi8BUt0ko2Pd8VM4wNfLNDHu j78DMu7E87MzBLazDble0QJxc20ulMTcU2eOowgWNOvNV8uKFngCHMKouYcmz4uEJ65pS vM1G/2SlNGY1yzUg1fG3Gcws1CbCvkcUD1iQAo8e5pEuIGY6+jCryVCKthuvxMEJMm74M Ur7ajqrbYz61vOrbaQfQ9uIy+AvNoYDUxH6Pc8SfPc6T9NlTM1fPo+u7HI2JQcZDXxzYy WlchmRg7exMwv66UFgmstlSxJqdTxpbnGH6+iJbiIbKuD8+dH+goM9UCbD0uK9X/zow== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793557139; h=Received:DKIM-Signature:Message-ID:Date:MIME-Version:User-Agent: Subject:From:To:Cc:References:Content-Language:Autocrypt:In-Reply-To: Content-Type:Content-Transfer-Encoding; bh=Ufkw0mhxgeNgH2/pQ2q7PPe3Br8wC4VML0hZBdX1Pi8=; b=SG/T/ksRFwv1tuQzEwl9ZCFqTwpi2ikIDp0e0x2MKvRjfBBqbasTJgZnB1UGq0X/p8I4 +/HiLh+WjxZaZTgVliJbPpYkPXGGuZObp5LtjfkvfNIdJQ5BlLMxpSUQIp7AfdiY8Zhnz ADpOvNwg4sWzcVDqUQa5iMaS1NgHaxOzPUK5BoagZ35FlN5UzIV36jEbld/eP7bHagP15 XTeLJq2/USYwdKrbbcQLqP41j0P+uPTB7tn/hKnIT0gKeEqDEn267VPVA+MTQV9xW95pq l8M+RN2tXkeymQN5BN01mb6M/7DXZ84Kp3gKfIfrR+5+VWC4ll3DMKIfH3dmSfemIGQ== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=klz8DA2W; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 44A5C606A324; Sun, 1 Nov 2026 18:18:58 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=klz8DA2W Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2ECA31F000C1; Sun, 1 Nov 2026 18:18:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793557138; bh=Ufkw0mhxgeNgH2/pQ2q7PPe3Br8wC4VML0hZBdX1Pi8=; h=Date:Subject:From:To:Cc:References:In-Reply-To; b=klz8DA2Wy4v8itPMM2Sk9hjHDXFe4ZdxZ7Wm3kSIbNIjgq2j0zYuwD3dHhzLIsL0e 4zKr0h4je0AERM9ZjdXzQgALGxnA98fIsYFgDKnS+aeSps60tnSRzpaw2RJNLqRveY 5tSqwghmdVskVhQLiTUVhkKxF+1lg2G48aFjA+jvhYGJnlq4x/ruqGjVl8SSILlFJ7 t4XRAy1rUINCeozRiHO9sXtJmZuofyoT4naAPHaZaTGPifbJmkANfnGNglb+yUCEGZ AeS0wXa8ZCWPh18XTbEWZyvoDH8pgu5Y2P7HBtGjqfkG+FTJhEtIpKe5JAFxSMAIkI bF9wyBE8cQqEA== Message-ID: Date: Sun, 1 Nov 2026 19:18:52 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] fs/proc/task_mmu: check THP exclusivity for each subpage From: "David Hildenbrand (Arm)" To: Kelvin Zhang , Andrew Morton , linux-mm@kvack.org Cc: "Liam R . Howlett" , Lorenzo Stoakes , Vlastimil Babka , Jann Horn , Pedro Falcato , Kalesh Singh , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, stable@vger.kernel.org References: <4c1b393a4b04bd94e4f1c7879ae04e1793eb0931.1793552446.git.zhangxp1998@gmail.com> <54617273-262f-4e17-b912-8b07113ca45c@kernel.org> Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <54617273-262f-4e17-b912-8b07113ca45c@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/1/26 19:07, David Hildenbrand (Arm) wrote: > On 11/1/26 18:01, Kelvin Zhang wrote: >> A PMD-mapped THP can contain subpages with different mapcounts. For >> example, after fork(), the child can unmap only part of the THP. The >> parent retains its PMD mapping, while the remaining child mappings >> are PTE mappings. >> >> pagemap_pmd_range_thp() passes the same initial page to >> __folio_page_mapped_exclusively() for every pagemap entry. With >> CONFIG_PAGE_MAPCOUNT=y, this applies the first subpage's exclusivity >> to all entries, falsely setting or clearing PM_MMAP_EXCLUSIVE for >> subpages with different sharing. >> >> Pass page + idx to check the subpage corresponding to each entry, as >> was done before converting to __folio_page_mapped_exclusively(). The >> index already accounts for reads starting partway through the PMD. >> The conservative whole-folio check without CONFIG_PAGE_MAPCOUNT is >> unchanged. >> >> Fixes: eb16876971ea ("fs/proc/task_mmu: remove per-page mapcount dependency for PM_MMAP_EXCLUSIVE (CONFIG_NO_PAGE_MAPCOUNT)") >> Cc: stable@vger.kernel.org # 6.15+ >> Signed-off-by: Kelvin Zhang >> --- >> Tested on x86-64 under QEMU/KVM with CONFIG_TRANSPARENT_HUGEPAGE=y >> and CONFIG_PAGE_MAPCOUNT=y, using the same test binary before and after >> this change. After fork(), the child unmaps the first, second, middle, >> or last 4 KiB subpage, while the parent retains a PMD mapping. Both >> bulk pagemap reads and reads starting inside the PMD are checked. >> >> Across three repetitions (12 cases), the unmodified kernel fails 24 of >> 84 assertions; with this change all 84 pass. Fully private and fully >> shared PMD controls pass before and after the fix. >> > > How was this found. Through an LLM or because there was actually a problem in > practice? > > Why do you care about this information to be reliable. > > If the answer is "I don't care, I don't care, an LLM reported so I blindly > submit a fix" we might just perform the switch to the CONFIG_NO_PAGE_MAPCOUNT > variant directly. > > > https://lore.kernel.org/linux-mm/20260412-mapcount-v1-4-05e8dfab52e0@kernel.org/ Sent my mail a bit to soon: the reason is that this might be one of the best-case scenarios: we broke something by accident, nobody cared for various releases; so now we can just change it. Thanks for the report! -- Cheers, David