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=1793602877; b=Vgsb1Rt2qJk7svrfKHWoU49m8bZkdAeEMNBnQ3RGYp7b13MsbbM0fMx8KN7l9jvS0TXp j1eXCecZgfaIPPIKcHSuEBW4sz8kTtaKSaA7V7TJPMeq50V8x7g7NOQ8C3MZX7Z+ZHDMr xmIyS7v9ZFRyOY0JPbprX4x19C0fRWXEZaDsrRyq2ZKLoFQc5074hXgu+11+yNtjPoEYX H5e9nQE0pfbhzp4wGpN6BYglccF+N0GBYDnCAQZD4bqQLad8RgMedM6BaPTnOgnVsHW9l 8UM0vJBcZnaC1XCBSo+C01HsqIeyJBMTFI/A7CbJ46LtHKVKz/wpMpvN8XgDcJFmkZA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793602877; 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=8jY8XpaYgmA53HXY22kW+XbJRfVwUAnog+5j0SSxl6A=; b=JGcOXJQE7z4+uVqeLnzyxzj6xnxhUPtLAf+QVF1K3ezxvzs2NuUwZoHjIlTIYsMWOKkz e/JmZzwS8VMJJqLOVMNrnzcx57P1fOOSIuexBCCeq4sHe+assKN8a8yL7D/ZOjnU9n/02 DKC5DmDR+tSAMU6nH+r7hJaEbcM2hy42xlKAIYgxAt9Jwmn98SceU4a8xRIhnHudxeasE PYW0hk5Rhj2gJPju68I3lZLVQTSkD/0bGMVw+3hko0bJAHDFNPLPYrsOI3eRTZ3s5HliF 6HWgr50hsAm9mqtfc+af+hXUdGXwd90p/bnmSrcDxMrcT6fjfXGwt6SjU7yBM+49wHA== 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=ayLpLZrD; 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 E92E8606A324; Mon, 2 Nov 2026 07:01:16 +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=ayLpLZrD Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4D1BF1F000C1; Mon, 2 Nov 2026 07:01:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793602876; bh=8jY8XpaYgmA53HXY22kW+XbJRfVwUAnog+5j0SSxl6A=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=ayLpLZrD91MV+ICG3Fack6rUmwXp0VVRmmG7WDL665UIqgU6aQAHE2cfgeEAFj2w6 3OfRdFnV9rNSF40NFSb/mC7TU6W9P2pqYZw4O6X9nx89FPsUKVy5Zk0oPjTnzja2CE j8zI7LfbKJL0GiikRYKCKlGArzfqQhnKzWAcab4bLDfFSLiajTkI7Hc1ePyxAzfG+h 2J+mqjiI3fgSphgmWft6k46KLM666AovCEVi+NwBQC9jRIKlAGgE1aTdGYQiylbMe7 2cq8J8kxXVr0UzQQa3PkEJUvDn1u51KL6XA2qoDuRecvHG2QH68KeiI0YA+ufzhGNA zQKGG44cskMxQ== Message-ID: <6b69aa22-f392-4cc8-9f7e-d9077e951cc4@kernel.org> Date: Mon, 2 Nov 2026 08:01:11 +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: Andrew Morton , Kelvin Zhang Cc: linux-mm@kvack.org, "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> <20261101110247.95a03a5601f235ffb39c54c1@linux-foundation.org> 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: <20261101110247.95a03a5601f235ffb39c54c1@linux-foundation.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/1/26 20:02, Andrew Morton wrote: > On Sun, 1 Nov 2026 09:01:00 -0800 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. > > Thanks. > > I don't want to pick on you at all, but this is a classic changelog - I > see this pattern so often. > > a) here's what the code does > > b) here's why that's wrong > > c) here's how I fixed it. > > But that's missing a very important bullet point: > > a) here's what the code does > > b) here's why that's wrong > > c) here's how this affects users > > d) here's how I fixed it. > > see? This omission is so common! And I might have sent one or two > emails asking people for the missing info. > > For the missing c), how does this sound? > > : With transparent huge pages, /proc/PID/pagemap can incorrectly > : report shared pages as exclusive, or vice versa, after fork() and > : partial unmapping. This can mislead memory-analysis and > : checkpoint/restore tools. Actual mappings and copy-on-write behavior > : are unaffected. > > And now that we have our statement of userspace impact, busy non-mm > downstream readers of this patch can easily see why your proposed > cc:stable is justified. > > > Also, it would be helpful to show brief before-and-after excerpts from > /proc/PID/pagemap to help others understand the impact of the change. > But I don't think we need a v2 just for this. See my reply. I'm not sure what to best do here, but I want to use this as an opportunity to say "see, nobody noticed in 1.5 years; and it was found be code analysis then, so let's simplify it". So it's either straight to [1] or a "let's do i first the old way" followed by [1]. I would just do [1] immediately. Because nobody cares about that flag. [1] https://lore.kernel.org/linux-mm/20260412-mapcount-v1-4-05e8dfab52e0@kernel.org/ -- Cheers, David