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 85E9C3DBA0 for ; Thu, 9 Apr 2026 12:29:44 +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=1775737784; cv=none; b=HBc5xZNvd86VNc4HNDobC5SBkXHm69XuCwa3KbYQ/OP7TYdv9v6U9W4dZzrOJYvoHi1TUjLIHxpRCktzN85cNpFsGJ6X9i69m4ayv6prrpHCxMiHx8ebvvHJH/tR2o/SwjKPe94MIoLsuypJs9KdGkPUTtzg7zPjKuTs91AQf7k= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775737784; c=relaxed/simple; bh=o1pnMljy3NO/nviBZv7VCCVCA0U1jFakSHYePKGRrTU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=JNxhl0G4EygemvVW0mrpAv8Po16gMNkbsOWiFGEQYjErWT6XeqiUgAwsnbuLp9/jUCsKQsc0XqmA/VislVC+h5/LDKXhaPacq6Jle09VvrsbmO3ZQkF6faeeJYKoRJ3nChBNFrxo5QrCwFBOEUc+kaigpajETDGhHbRUytMot8c= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=qn4LlS61; 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="qn4LlS61" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 36AACC4CEF7; Thu, 9 Apr 2026 12:29:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775737784; bh=o1pnMljy3NO/nviBZv7VCCVCA0U1jFakSHYePKGRrTU=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=qn4LlS61va7HSew9APqWOk+ljAjB4T+WrPyHcr1pPv5Fu8i7v+r9N8X+tpVGOGvh8 5UqhPhFyISEJwa2k2FZEPVmfoDkmiEQYRPyNII9M/2fqLSD9mHdbcoZurS51N681r+ OVYK+/3jCRe/a3PO/QwbbqfCP8504dlfMFRwh7nG2d75jBPJhLPkuvfgfN+EciSgDw R40td0DHyVJQS8Tvivul7L6AZM0ND/jpJK29loRRr0VsXtwo00TETFDZXGgAlTMIW/ sNSndBPpNq1R0oB6m/QrlBqtI9d0u0/KxiNRdK1DH3kS12DS34viFkOMCbWvX9hVJC /gVUo7Mky5P8A== Message-ID: Date: Thu, 9 Apr 2026 14:29:38 +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: [RFC PATCH] mm/sparse: remove sparse_buffer To: Muchun Song Cc: Muchun Song , Andrew Morton , yinghai@kernel.org, Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260407083951.2823915-1-songmuchun@bytedance.com> <70EF8E41-31A2-4B43-BABE-7218FD5F7271@linux.dev> 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: <70EF8E41-31A2-4B43-BABE-7218FD5F7271@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 4/9/26 13:40, Muchun Song wrote: > > >> On Apr 8, 2026, at 21:40, David Hildenbrand (Arm) wrote: >> >> On 4/7/26 10:39, Muchun Song wrote: >>> The sparse_buffer was originally introduced in commit 9bdac9142407 >>> ("sparsemem: Put mem map for one node together.") to allocate a >>> contiguous block of memory for all memmaps of a NUMA node. >>> >>> However, the original commit message did not clearly state the actual >>> benefits or the necessity of keeping all memmap areas strictly >>> contiguous for a given node. >> >> We don't want the memmap to be scattered around, given that it is one of >> the biggest allocations during boot. >> >> It's related to not turning too many memory blocks/sections >> un-offlinable I think. > > Hi David, > > Got it. > >> >> I always imagined that memblock would still keep these allocations close >> to each other. Can you verify if that is indeed true? > > You raised a very interesting point about whether memblock keeps > these allocations close to each other. I've done a thorough test > on a 16GB VM by printing the actual physical allocations. > > I enabled the existing debug logs in arch/x86/mm/init_64.c to > trace the vmemmap_set_pmd allocations. Here is what really happens: > > When using vmemmap_alloc_block without sparse_buffer, the > memblock allocator allocates 2MB chunks. Because memblock > allocates top-down by default, the physical allocations look > like this: > > [ffe6475cc0000000-ffe6475cc01fffff] PMD -> [ff3cb082bfc00000-ff3cb082bfdfffff] on node 0 > [ffe6475cc0200000-ffe6475cc03fffff] PMD -> [ff3cb082bfa00000-ff3cb082bfbfffff] on node 0 > [ffe6475cc0400000-ffe6475cc05fffff] PMD -> [ff3cb082bf800000-ff3cb082bf9fffff] on node 0 > [ffe6475cc0600000-ffe6475cc07fffff] PMD -> [ff3cb082bf600000-ff3cb082bf7fffff] on node 0 > [ffe6475cc0800000-ffe6475cc09fffff] PMD -> [ff3cb082bf400000-ff3cb082bf5fffff] on node 0 > [ffe6475cc0a00000-ffe6475cc0bfffff] PMD -> [ff3cb082bf200000-ff3cb082bf3fffff] on node 0 > [ffe6475cc0c00000-ffe6475cc0dfffff] PMD -> [ff3cb082bf000000-ff3cb082bf1fffff] on node 0 > [ffe6475cc0e00000-ffe6475cc0ffffff] PMD -> [ff3cb082bee00000-ff3cb082beffffff] on node 0 > [ffe6475cc1000000-ffe6475cc11fffff] PMD -> [ff3cb082bec00000-ff3cb082bedfffff] on node 0 > [ffe6475cc1200000-ffe6475cc13fffff] PMD -> [ff3cb082bea00000-ff3cb082bebfffff] on node 0 > [ffe6475cc1400000-ffe6475cc15fffff] PMD -> [ff3cb082be800000-ff3cb082be9fffff] on node 0 > [ffe6475cc1600000-ffe6475cc17fffff] PMD -> [ff3cb082be600000-ff3cb082be7fffff] on node 0 > [ffe6475cc1800000-ffe6475cc19fffff] PMD -> [ff3cb082be400000-ff3cb082be5fffff] on node 0 > [ffe6475cc1a00000-ffe6475cc1bfffff] PMD -> [ff3cb082be200000-ff3cb082be3fffff] on node 0 > [ffe6475cc1c00000-ffe6475cc1dfffff] PMD -> [ff3cb082be000000-ff3cb082be1fffff] on node 0 > [ffe6475cc1e00000-ffe6475cc1ffffff] PMD -> [ff3cb082bde00000-ff3cb082bdffffff] on node 0 > [ffe6475cc2000000-ffe6475cc21fffff] PMD -> [ff3cb082bdc00000-ff3cb082bddfffff] on node 0 > [ffe6475cc2200000-ffe6475cc23fffff] PMD -> [ff3cb082bda00000-ff3cb082bdbfffff] on node 0 > [ffe6475cc2400000-ffe6475cc25fffff] PMD -> [ff3cb082bd800000-ff3cb082bd9fffff] on node 0 > [ffe6475cc2600000-ffe6475cc27fffff] PMD -> [ff3cb082bd600000-ff3cb082bd7fffff] on node 0 > [ffe6475cc2800000-ffe6475cc29fffff] PMD -> [ff3cb082bd400000-ff3cb082bd5fffff] on node 0 > [ffe6475cc2a00000-ffe6475cc2bfffff] PMD -> [ff3cb082bd200000-ff3cb082bd3fffff] on node 0 > [ffe6475cc2c00000-ffe6475cc2dfffff] PMD -> [ff3cb082bd000000-ff3cb082bd1fffff] on node 0 > [ffe6475cc2e00000-ffe6475cc2ffffff] PMD -> [ff3cb082bce00000-ff3cb082bcffffff] on node 0 > [ffe6475cc4000000-ffe6475cc41fffff] PMD -> [ff3cb082bcc00000-ff3cb082bcdfffff] on node 0 > [ffe6475cc4200000-ffe6475cc43fffff] PMD -> [ff3cb082bca00000-ff3cb082bcbfffff] on node 0 > [ffe6475cc4400000-ffe6475cc45fffff] PMD -> [ff3cb082bc800000-ff3cb082bc9fffff] on node 0 > [ffe6475cc4600000-ffe6475cc47fffff] PMD -> [ff3cb082bc600000-ff3cb082bc7fffff] on node 0 > [ffe6475cc4800000-ffe6475cc49fffff] PMD -> [ff3cb082bc400000-ff3cb082bc5fffff] on node 0 > [ffe6475cc4a00000-ffe6475cc4bfffff] PMD -> [ff3cb082bc200000-ff3cb082bc3fffff] on node 0 > [ffe6475cc4c00000-ffe6475cc4dfffff] PMD -> [ff3cb082bc000000-ff3cb082bc1fffff] on node 0 > [ffe6475cc4e00000-ffe6475cc4ffffff] PMD -> [ff3cb082bbe00000-ff3cb082bbffffff] on node 0 > [ffe6475cc5000000-ffe6475cc51fffff] PMD -> [ff3cb083bfa00000-ff3cb083bfbfffff] on node 1 > [ffe6475cc5200000-ffe6475cc53fffff] PMD -> [ff3cb083bf800000-ff3cb083bf9fffff] on node 1 > [ffe6475cc5400000-ffe6475cc55fffff] PMD -> [ff3cb083bf600000-ff3cb083bf7fffff] on node 1 > [ffe6475cc5600000-ffe6475cc57fffff] PMD -> [ff3cb083bf400000-ff3cb083bf5fffff] on node 1 > [ffe6475cc5800000-ffe6475cc59fffff] PMD -> [ff3cb083bf200000-ff3cb083bf3fffff] on node 1 > [ffe6475cc5a00000-ffe6475cc5bfffff] PMD -> [ff3cb083bf000000-ff3cb083bf1fffff] on node 1 > [ffe6475cc5c00000-ffe6475cc5dfffff] PMD -> [ff3cb083b6e00000-ff3cb083b6ffffff] on node 1 > [ffe6475cc5e00000-ffe6475cc5ffffff] PMD -> [ff3cb083b6c00000-ff3cb083b6dfffff] on node 1 > [ffe6475cc6000000-ffe6475cc61fffff] PMD -> [ff3cb083b6a00000-ff3cb083b6bfffff] on node 1 > [ffe6475cc6200000-ffe6475cc63fffff] PMD -> [ff3cb083b6800000-ff3cb083b69fffff] on node 1 > [ffe6475cc6400000-ffe6475cc65fffff] PMD -> [ff3cb083b6600000-ff3cb083b67fffff] on node 1 > [ffe6475cc6600000-ffe6475cc67fffff] PMD -> [ff3cb083b6400000-ff3cb083b65fffff] on node 1 > [ffe6475cc6800000-ffe6475cc69fffff] PMD -> [ff3cb083b6200000-ff3cb083b63fffff] on node 1 > [ffe6475cc6a00000-ffe6475cc6bfffff] PMD -> [ff3cb083b6000000-ff3cb083b61fffff] on node 1 > [ffe6475cc6c00000-ffe6475cc6dfffff] PMD -> [ff3cb083b5e00000-ff3cb083b5ffffff] on node 1 > [ffe6475cc6e00000-ffe6475cc6ffffff] PMD -> [ff3cb083b5c00000-ff3cb083b5dfffff] on node 1 > [ffe6475cc7000000-ffe6475cc71fffff] PMD -> [ff3cb083b5a00000-ff3cb083b5bfffff] on node 1 > [ffe6475cc7200000-ffe6475cc73fffff] PMD -> [ff3cb083b5800000-ff3cb083b59fffff] on node 1 > [ffe6475cc7400000-ffe6475cc75fffff] PMD -> [ff3cb083b5600000-ff3cb083b57fffff] on node 1 > [ffe6475cc7600000-ffe6475cc77fffff] PMD -> [ff3cb083b5400000-ff3cb083b55fffff] on node 1 > [ffe6475cc7800000-ffe6475cc79fffff] PMD -> [ff3cb083b5200000-ff3cb083b53fffff] on node 1 > [ffe6475cc7a00000-ffe6475cc7bfffff] PMD -> [ff3cb083b5000000-ff3cb083b51fffff] on node 1 > [ffe6475cc7c00000-ffe6475cc7dfffff] PMD -> [ff3cb083b4e00000-ff3cb083b4ffffff] on node 1 > [ffe6475cc7e00000-ffe6475cc7ffffff] PMD -> [ff3cb083b4c00000-ff3cb083b4dfffff] on node 1 > [ffe6475cc8000000-ffe6475cc81fffff] PMD -> [ff3cb083b4a00000-ff3cb083b4bfffff] on node 1 > [ffe6475cc8200000-ffe6475cc83fffff] PMD -> [ff3cb083b4800000-ff3cb083b49fffff] on node 1 > [ffe6475cc8400000-ffe6475cc85fffff] PMD -> [ff3cb083b4600000-ff3cb083b47fffff] on node 1 > [ffe6475cc8600000-ffe6475cc87fffff] PMD -> [ff3cb083b4400000-ff3cb083b45fffff] on node 1 > [ffe6475cc8800000-ffe6475cc89fffff] PMD -> [ff3cb083b4200000-ff3cb083b43fffff] on node 1 > [ffe6475cc8a00000-ffe6475cc8bfffff] PMD -> [ff3cb083b4000000-ff3cb083b41fffff] on node 1 > [ffe6475cc8c00000-ffe6475cc8dfffff] PMD -> [ff3cb083b3e00000-ff3cb083b3ffffff] on node 1 > [ffe6475cc8e00000-ffe6475cc8ffffff] PMD -> [ff3cb083b3c00000-ff3cb083b3dfffff] on node 1 > > Notice that the physical chunks are strictly adjacent to each > other, but in descending order! > > So, they are NOT "scattered around" the whole node randomly. > Instead, they are packed densely back-to-back in a single > contiguous physical range (just mapped top-down in 2MB pieces). > > Because they are packed tightly together within the same > contiguous physical memory range, they will at most consume or > pollute the exact same number of memory blocks as a single > contiguous allocation (like sparse_buffer did). Therefore, this > will NOT turn additional memory blocks/sections into an > "un-offlinable" state. > > It seems we can safely remove the sparse buffer preallocation > mechanism, don't you think? Yes, what I suspected. Is there a performance implication when doing many individual memmap_alloc(), for example, on a larger system with many sections? -- Cheers, David