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=1793556475; b=RkrRtcIwDli5Tb7OatMU4vG5dtauaClF5F4WBU/l5JiY46gKC5s2XWWpi2JyI9lRIxRR iL1j0mmNZxBAep81TlfuhEQ5kihE85XZCxTBFynkDekhvrXLR2ENBxzAtlQ53MJs4UVoF tdZA3frrc2xgZIIrOy+4wpLdSwMdVqHpXqq4D0l20VVg+mJF3jfV3SS0/We3KeLGuMa3J rT8+/kRWmHwCsJxSMFr2F8HAHqtonV0CPb1HAW83FoYTYsA1OPE3NoIyqlGXEJEfxhbOm IR7fJ8GU9bn+nH6pMCiP6N84PIQbT5DeWf91jvK1fWcF36tr9ta/zEAVDLr5WLh4XqA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793556475; h=Received:DKIM-Signature:Message-ID:Date:MIME-Version:User-Agent: Subject:To:Cc:References:From:Content-Language:Autocrypt:In-Reply-To: Content-Type:Content-Transfer-Encoding; bh=wtVX074xn4Ax9Ym2W5G9udDOf7jc7SoXAtiT9SubY18=; b=uEJbrJLU9m2w68KNEI7WL9YZVYAXRYI70pyEywBOrBAFqnh8M3cA7qWXlyHPRYSpNp80 yYMCgJpitcMy1CDMsiWdjrrTUpmwhbHnqxsdDRfBz+/ZfepBqQkRkanf+sI5KfbAUIHrg LA38CIy0p+S9z3RQd550KaKc+hRqgZjZKuyIvUw+kAsWZjbkhnYt50Z3NZdDewHYhOvob ELC8MxHmt6hrp0H3SLKtT+FtYQ3pgubDQNIl5M7U+CZI4sNd9OEWwp4p8oxPAxfcPXTq2 jyePAagXZbEog85Ar3tCGsQniQQOOgfNd+Ddgk7+GEqD+stBg+PVonEndqpMXlVpeRQ== 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=h1TzP+oE; 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 12FF2606A324; Sun, 1 Nov 2026 18:07:54 +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=h1TzP+oE Received: by smtp.kernel.org (Postfix) with ESMTPSA id CCB141F000C1; Sun, 1 Nov 2026 18:07:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793556474; bh=wtVX074xn4Ax9Ym2W5G9udDOf7jc7SoXAtiT9SubY18=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=h1TzP+oEyvUummX3FBrsaDg6F/T4dtjOssjippILdgLc7Pshpgl+k71VVqaBz6gjZ 00qb4JZ7RpQTw0jGCW58RLWfO0T5DtrzR6+rMbQoflSGjSFxieCo+wMTb+EJjHvCi5 eIQtv8k/EQhMV6LSRI7w+VcB8+BQz/JoJ0xXSate5ZZjFePTzGCy21AkAtA0p9FizX oXzFJvR08nY5j30ATR+qwIwRpHL1JHdIHYUoH6sbE1KL/lCb78SSctsV9H1wDwCQ49 DOqFTmi2w9Ews2UgoYCcuMGExZtqCXiMR5A6SVQ6/GPcu92wLeFjhk3KCkrIXrwD7I lDYKLzlxbvAqA== Message-ID: <54617273-262f-4e17-b912-8b07113ca45c@kernel.org> Date: Sun, 1 Nov 2026 19:07:49 +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 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> From: "David Hildenbrand (Arm)" 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: <4c1b393a4b04bd94e4f1c7879ae04e1793eb0931.1793552446.git.zhangxp1998@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit 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/ -- Cheers, David