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 E24C41AE877; Fri, 20 Mar 2026 18:49:52 +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=1774032593; cv=none; b=dizVVQqEpA7xL5xSWSrS7JN8q9/XMOUgWIIeifYLbyafJsJ9TZQ9gETuUZQ72VxfNaDUvtJQHwGuelSsWgvGL3iI0gzQ1/t2LJY5mqbFiwYEIlcpascnaq48XbNa2eojrkcMzWkuD0DCkMyCTifDNT0VsmFpNO/YBug7/XEob8A= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774032593; c=relaxed/simple; bh=CZPats9e/xIykQ0p6beR56p5pT3SBFNJcBRMlbIFXJ8=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=I8xURTFaBe+SVg1dmpb3oD1jLe+6tAM6smE4P1t45/055N3cuq2pS4joORjLlkpZtbhZjAlJ5c/qgd/IkKIgJb7nC1drBrGCO+pJ7B38ZqU01s2VwdjcLy1kOYm+IwYTfU5orPCSUzuH88mW2MbaNDISG0dgI+TGwWN0dpQao7I= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=HgPgwcmz; 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="HgPgwcmz" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5F1EBC4CEF7; Fri, 20 Mar 2026 18:49:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1774032592; bh=CZPats9e/xIykQ0p6beR56p5pT3SBFNJcBRMlbIFXJ8=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=HgPgwcmzuu4IHGVR8cx6TSvo6cBfxT/Zora5SGzhglKRzYmLMnRkAp41Lu3rXXGDe CQvykMQFTA/jVXGx50Krg5uoA42bhJCh4B8fAs84jFW7Qa233Fl2U4Fpj5cMp1rPtW ix2NSc1xwV7NR3Ed3qcs5FwIgNYlNc59eiirnv1REb9SWMbAMEeozSLs/6tTbBoQfa AsjqG2STRS1m24JkkjnL+3q2UBEIghZOY6qkRpZ/2LVckJDPQ+BYXfbAh/ZHxfFvJm H0iSTwulSmW0juJMkizsWIkqiKoI22h1WLsAi9aZzdMWnNmE/adMLK3CU0n6mc3vov wYcYxyl/xzwiw== Message-ID: <306f0fd2-cba1-4f48-af05-248be2bc9506@kernel.org> Date: Fri, 20 Mar 2026 19:49:46 +0100 Precedence: bulk X-Mailing-List: linux-cxl@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 09/14] mm/sparse: remove CONFIG_MEMORY_HOTPLUG-specific usemap allocation handling To: "Lorenzo Stoakes (Oracle)" Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-cxl@vger.kernel.org, Andrew Morton , Oscar Salvador , Axel Rasmussen , Yuanchu Xie , Wei Xu , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko References: <20260317165652.99114-1-david@kernel.org> <20260317165652.99114-10-david@kernel.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: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 3/17/26 20:48, Lorenzo Stoakes (Oracle) wrote: > On Tue, Mar 17, 2026 at 05:56:47PM +0100, David Hildenbrand (Arm) wrote: >> In 2008, we added through commit 48c906823f39 ("memory hotplug: allocate >> usemap on the section with pgdat") quite some complexity to try >> allocating memory for the "usemap" (storing pageblock information >> per memory section) for a memory section close to the memory of the >> "pgdat" of the node. >> >> The goal was to make memory hotunplug of boot memory more likely to >> succeed. That commit also added some checks for circular dependencies >> between two memory sections, whereby two memory sections would contain >> each others usemap, turning bot memory sections un-removable. > > Typo: bot -> both. Presumably you are not talking about memory a bot of some > kind allocated :P > >> >> However, in 2010, commit a4322e1bad91 ("sparsemem: Put usemap for one node >> together") started allocating the usemap for multiple memory >> sections on the same node in one chunk, effectively grouping all usemap >> allocations of the same node in a single memblock allocation. >> >> We don't really give guarantees about memory hotunplug of boot memory, and >> with the change in 2010, it is pretty much impossible in practice to get >> any circular dependencies. > > Pretty much impossible? :) We can probably go so far as to so impossible no? Yes. > >> >> commit 48c906823f39 ("memory hotplug: allocate usemap on the section with >> pgdat") also added the comment: >> >> "Similarly, a pgdat can prevent a section being removed. If >> section A contains a pgdat and section B >> contains the usemap, both sections become inter-dependent." >> >> Given that we don't free the pgdat anymore, that comment (and handling) >> does not apply. > > Isn't pgdat synonymous with a node and that's the data structure that describes > a node right? Confusingly typedef'd from pglist_data to pg_data_t but then > referred to as pgdat because all that makes so much sense :) Yeah, in general we refer to the NODE_DATA as pgdat (grep for it and you'll be surprised). > > But I'm confused, does a section containing a pgdat mean a section having the > pgdat data structure literally allocated in it? Yes. "struct pgdat" placed in some memory section. > > A usemap is... something that tracks pageblock metadata I think right? Yes. Essentially a large array of bytes, whereby each byte describes a pageblock data (migratetype etc) > > Anyway I'm also confused by 'given we don't free the pgdat any more', but the > comment says a 'pgdat can prevent a section being removed' rather than anything > about it being removed? Well, if a pgdat resides in some memory section, given that it is unmovable turns the whole memory section unremovable -> hotunplug fails. Assuming you could free the pgdat when the node goes offlining, you would turn that memory section removable. And I think that commit somehow assumed that the last memory section could be removed if all it contains is the corresponding pgdat (which was never the case). > > I guess it means the OTHER section could be prevented from being removed even > after it's gone.. somehow? > > Anyway! I think maybe this could be clearer, somehow :) I'm afraid the whole purpose of the original patch was sketchy, which is also while I fail to even explain the original motivation clearly. Now it's fortunately no longer required. :) > >> >> So let's simply remove this complexity. >> >> Signed-off-by: David Hildenbrand (Arm) > > I think what you've done in the patch is right though, we're not doing any of > these dances after a4322e1bad91 and pgdats sitting around mean we don't really > care about where the usemap goes anyway I don't think so... > > I usemap and I find myself in a place where I give you a: > > Reviewed-by: Lorenzo Stoakes (Oracle) > Thanks ;) [...] >> - >> #ifdef CONFIG_SPARSEMEM_VMEMMAP >> unsigned long __init section_map_size(void) >> { >> @@ -486,7 +390,6 @@ void __init sparse_init_early_section(int nid, struct page *map, >> unsigned long pnum, unsigned long flags) >> { >> BUG_ON(!sparse_usagebuf || sparse_usagebuf >= sparse_usagebuf_end); >> - check_usemap_section_nr(nid, sparse_usagebuf); >> sparse_init_one_section(__nr_to_section(pnum), pnum, map, >> sparse_usagebuf, SECTION_IS_EARLY | flags); >> sparse_usagebuf = (void *)sparse_usagebuf + mem_section_usage_size(); >> @@ -497,8 +400,7 @@ static int __init sparse_usage_init(int nid, unsigned long map_count) >> unsigned long size; >> >> size = mem_section_usage_size() * map_count; >> - sparse_usagebuf = sparse_early_usemaps_alloc_pgdat_section( >> - NODE_DATA(nid), size); >> + sparse_usagebuf = memblock_alloc_node(size, SMP_CACHE_BYTES, nid); > > I guess nid here is the same node as the pgdat? Yes! before we used NODE_DATA(nid)->node_id, which is really just ... nid :) -- Cheers, David