From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 31419371D19 for ; Tue, 31 Mar 2026 20:05:03 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774987504; cv=none; b=Gwa9FtpgUIAo4+4iyRZFWsDBDciAhgorbWLBPLguD9lAfwUO7wpN1cklZN190ALC8QNbv0mc3wqf2/s19CW0Gt4tNgMUf0cFJjdt/WMx1YFJeoMlriq6xs3SVxC9LyqhDZXuzIonwnVVHPpK6HuIPU5bURjr+xGRWZvQeJ/Vevk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774987504; c=relaxed/simple; bh=p4iHbgiMDPcx1vmH5nX95S1Qm7dNTZ6vVmQCUSuJfCk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=fRQA6oqvGpTasNi4uYZBM46+BqBjNWyldlTxN/1AzZ2uiBh22kSgQBHHzrVuulBzUGmsYLs/sowd6HH5gNdRSOfa1ag3Xdq5i6k51DIMU4et6+k290I3wq4l126mBmMnyJwApUakkuF1zqTG0CeZtThbFnwu7RqlYZMSNvW20tM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=R94YMlP/; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="R94YMlP/" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1FF79C19423; Tue, 31 Mar 2026 20:04:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1774987503; bh=p4iHbgiMDPcx1vmH5nX95S1Qm7dNTZ6vVmQCUSuJfCk=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=R94YMlP/6+sA8K43jL2LcOK5OTgIssoC/au/el4KlnCmCHy26ih5mzqXN4myxpE85 c0nMCL2+p4iodkuzTmKEQv48Sgm8snhlUqo/oKekYWquLKA3cK+buVi5nGOzyIU3gL mfbubDsCzeKAdhVcvv0jp5+nU5NyQC2YiI22hImCYgNTwClbKEeyQ5dTondOvsaG7H PDawVk9eZVFpcw9TyG6r7nRnv5Jv3es69FrI7rsJ8Y6ktX3JBixIzkwQ+rQPjHeLHt w3/pdSuvQjhwHF10Sql5sFUw/JBIploDMSGoMp5ClfPfmffK1N1T6ZjQFkZd2IRDg7 DmS4U6pFsw0xQ== Message-ID: <29442561-9426-4635-9b7c-cdd138076127@kernel.org> Date: Tue, 31 Mar 2026 22:04:52 +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] mm/sparse: fix BUILD_BUG_ON check for section map alignment To: Andrew Morton , Muchun Song Cc: Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Petr Tesarik , linux-mm@kvack.org, linux-kernel@vger.kernel.org, muchun.song@linux.dev References: <20260331113023.2068075-1-songmuchun@bytedance.com> <20260331125529.de4deb7b46239fb13e4be517@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: <20260331125529.de4deb7b46239fb13e4be517@linux-foundation.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 3/31/26 21:55, Andrew Morton wrote: > On Tue, 31 Mar 2026 19:30:23 +0800 Muchun Song wrote: > >> The comment in mmzone.h states that the alignment requirement >> is the minimum of PAGE_SHIFT and PFN_SECTION_SHIFT. However, the >> pointer arithmetic (mem_map - section_nr_to_pfn()) results in >> a byte offset scaled by sizeof(struct page). Thus, the actual >> alignment provided by the second term is PFN_SECTION_SHIFT + >> __ffs(sizeof(struct page)). >> >> Update the compile-time check and the mmzone.h comment to >> accurately reflect this mathematically guaranteed alignment by >> taking the minimum of PAGE_SHIFT and PFN_SECTION_SHIFT + >> __ffs(sizeof(struct page)). This avoids the issue of the check >> being overly restrictive on architectures like powerpc where >> PFN_SECTION_SHIFT alone is very small (e.g., 6). >> >> Also, remove the exhaustive per-architecture bit-width list from the >> comment; such details risk falling out of date over time and may >> inadvertently be left un-updated, while the existing BUILD_BUG_ON >> provides sufficient compile-time verification of the constraint. >> >> No runtime impact so far: SECTION_MAP_LAST_BIT happens to fit within >> the smaller limit on all existing architectures. > > Thanks. So this can fix the build on some arch/kconfig combinations? > > Do you think we should fix older kernels? > >> Fixes: def9b71ee651 ("include/linux/mmzone.h: fix explanation of lower bits in the SPARSEMEM mem_map pointer") > > Eight years ago so I'm going with "no". IIUC, there is nothing broken. Could only be revealed by some new architecture. So no stable :) -- Cheers, David