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 1A55B37104D for ; Mon, 9 Feb 2026 11:38:27 +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=1770637108; cv=none; b=PF0c87YaNUpFmGq4ewXn24xtNEb/4X5qKgQxYgHM5KtPYLAyA/knWT3GMIuT3goyMLmxRc3xzwwugJ/5xEn298PAAdXdieJoaLknCQmQ19GPYdW6npmSzWi7WrxwrSf7MCsN3Vc9AIikBFXQOIj9VwMHqi/MrydDsZ3Uzqn8A/8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770637108; c=relaxed/simple; bh=Yda36AuZ+02mfV4OjNuEZ6kGbLXGT4SJGq10pr9YUhM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=BxRuiNapRYgYwpnxxzRm7QlCq17ds/q3h/gGwXcui5N36+GN0KkbrUKCMOScGsbUn+fEIl4GSFh2a0FX8UzAghPcYXYb+CPin07u0QK3rd3RBPJre4IsJt5pKqLNUITtKKIpUDTSDyCSYInKopqp2Jm0QZer2iUcZnvzmNd4m6A= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=LisPAGjv; 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="LisPAGjv" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 389C0C116C6; Mon, 9 Feb 2026 11:38:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1770637107; bh=Yda36AuZ+02mfV4OjNuEZ6kGbLXGT4SJGq10pr9YUhM=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=LisPAGjvsYTz3BdQyeJf4wUIBxZzUdSBykWdzFYDl86yAqqj10Np62aW9iUKw+ikj CBNMJmBbA6OSQihawVjSgfd75gxSUwLLUIAIrFBUQ3YzAUB8GzI2vI9i6hB5WPaSdd 8R4aO40mVoh7nTkXVQuYmaYV+P4mjdwe5b/z5ekOnlcGybeKOrqXcQJMsypF+F9MMu kDmZGFmWVSUdh5UVJI0CK3JycN1ifWVMRDOeXJk+FYbgsDVh4HIAclXNk9tcYF4/EZ ikuggntFLU+YJyKmXsskitJody8K6iLX1177A7tSYSTQRdycvRERNbhVQoUH4iF+zd GZrnundvLjt1A== Message-ID: <79272ca0-a326-4c0d-9e18-eea8e2d68160@kernel.org> Date: Mon, 9 Feb 2026 12:38:20 +0100 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 v9 2/2] mm/memory hotplug/unplug: Optimize zone->contiguous update when changes pfn range To: Mike Rapoport Cc: Tianyou Li , Oscar Salvador , Wei Yang , Michal Hocko , linux-mm@kvack.org, Yong Hu , Nanhai Zou , Yuan Liu , Tim Chen , Qiuxu Zhuo , Yu C Chen , Pan Deng , Chen Zhang , linux-kernel@vger.kernel.org References: <20260130163756.2674225-1-tianyou.li@intel.com> <20260130163756.2674225-3-tianyou.li@intel.com> <3cb317fa-abe0-4946-9f00-da00bade2def@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; format=flowed Content-Transfer-Encoding: 7bit On 2/8/26 20:39, Mike Rapoport wrote: > On Sat, Feb 07, 2026 at 12:00:09PM +0100, David Hildenbrand (Arm) wrote: >> On 1/30/26 17:37, Tianyou Li wrote: >>> When invoke move_pfn_range_to_zone or remove_pfn_range_from_zone, it will >>> update the zone->contiguous by checking the new zone's pfn range from the >>> beginning to the end, regardless the previous state of the old zone. When >>> the zone's pfn range is large, the cost of traversing the pfn range to >>> update the zone->contiguous could be significant. >>> >>> Add fast paths to quickly detect cases where zone is definitely not >>> contiguous without scanning the new zone. The cases are: when the new range >>> did not overlap with previous range, the contiguous should be false; if the >>> new range adjacent with the previous range, just need to check the new >>> range; if the new added pages could not fill the hole of previous zone, the >>> contiguous should be false. >>> >>> The following test cases of memory hotplug for a VM [1], tested in the >>> environment [2], show that this optimization can significantly reduce the >>> memory hotplug time [3]. >>> >>> +----------------+------+---------------+--------------+----------------+ >>> | | Size | Time (before) | Time (after) | Time Reduction | >>> | +------+---------------+--------------+----------------+ >>> | Plug Memory | 256G | 10s | 2s | 80% | >>> | +------+---------------+--------------+----------------+ >>> | | 512G | 33s | 6s | 81% | >>> +----------------+------+---------------+--------------+----------------+ >>> >>> +----------------+------+---------------+--------------+----------------+ >>> | | Size | Time (before) | Time (after) | Time Reduction | >>> | +------+---------------+--------------+----------------+ >>> | Unplug Memory | 256G | 10s | 2s | 80% | >>> | +------+---------------+--------------+----------------+ >>> | | 512G | 34s | 6s | 82% | >>> +----------------+------+---------------+--------------+----------------+ >>> >>> [1] Qemu commands to hotplug 256G/512G memory for a VM: >>> object_add memory-backend-ram,id=hotmem0,size=256G/512G,share=on >>> device_add virtio-mem-pci,id=vmem1,memdev=hotmem0,bus=port1 >>> qom-set vmem1 requested-size 256G/512G (Plug Memory) >>> qom-set vmem1 requested-size 0G (Unplug Memory) >>> >>> [2] Hardware : Intel Icelake server >>> Guest Kernel : v6.18-rc2 >>> Qemu : v9.0.0 >>> >>> Launch VM : >>> qemu-system-x86_64 -accel kvm -cpu host \ >>> -drive file=./Centos10_cloud.qcow2,format=qcow2,if=virtio \ >>> -drive file=./seed.img,format=raw,if=virtio \ >>> -smp 3,cores=3,threads=1,sockets=1,maxcpus=3 \ >>> -m 2G,slots=10,maxmem=2052472M \ >>> -device pcie-root-port,id=port1,bus=pcie.0,slot=1,multifunction=on \ >>> -device pcie-root-port,id=port2,bus=pcie.0,slot=2 \ >>> -nographic -machine q35 \ >>> -nic user,hostfwd=tcp::3000-:22 >>> >>> Guest kernel auto-onlines newly added memory blocks: >>> echo online > /sys/devices/system/memory/auto_online_blocks >>> >>> [3] The time from typing the QEMU commands in [1] to when the output of >>> 'grep MemTotal /proc/meminfo' on Guest reflects that all hotplugged >>> memory is recognized. >>> >>> Reported-by: Nanhai Zou >>> Reported-by: Chen Zhang >>> Tested-by: Yuan Liu >>> Reviewed-by: Tim Chen >>> Reviewed-by: Qiuxu Zhuo >>> Reviewed-by: Yu C Chen >>> Reviewed-by: Pan Deng >>> Reviewed-by: Nanhai Zou >>> Reviewed-by: Yuan Liu >>> Signed-off-by: Tianyou Li >>> --- >> >> Thanks for all your work on this and sorry for being slower with >> review the last month. >> >> While I was in the shower I was thinking about how much I hate >> zone->contiguous + the pageblock walking, and how we could just get >> rid of it. >> >> You know, just what you do while having a relaxing shower. >> >> >> And I was wondering: >> >> (a) in which case would we have zone_spanned_pages == zone_present_pages >> and the zone *not* being contiguous? I assume this just cannot happen, >> otherwise BUG. >> >> (b) in which case would we have zone_spanned_pages != zone_present_pages >> and the zone *being* contiguous? I assume in some cases where we have small >> holes within a pageblock? >> >> Reading the doc of __pageblock_pfn_to_page(), there are some weird >> scenarios with holes in pageblocks. > > It seems that "zone->contigous" is really bad name for what this thing > represents. > > tl;dr I don't think zone_spanned_pages == zone_present_pages is related to > zone->contigous at all :) > > If you look at pageblock_pfn_to_page() and __pageblock_pfn_to_page(), the > check for zone->contigous should guarantee that the entire pageblock has a > valid memory map and that the entire pageblock fits a zone and does not > cross zone/node boundaries. > > For coldplug memory the memory map is valid for every section that has > present memory, i.e. even it there is a hole in a section, it's memory map > will be populated and will have struct pages. > > When zone->contigous is false, the slow path in __pageblock_pfn_to_page() > essentially checks if the first page in a pageblock is online and if first > and last pages are in the zone being compacted. > > AFAIU, in the hotplug case an entire pageblock is always onlined to the > same zone, so zone->contigous won't change after the hotplug is complete. > > We might set it to false in the beginning of the hotplug to avoid scanning > offline pages, although I'm not sure if it's possible. > > But in the end of hotplug we can simply restore the old value and move on. > > For the coldplug case I'm also not sure it's worth the hassle, we could > just let compaction scan a few more pfns for those rare weird pageblocks > and bail out on wrong page conditions. > >> I.e., on my notebook I have >> >> $ cat /proc/zoneinfo | grep -E "Node|spanned|present" >> Node 0, zone DMA >> spanned 4095 >> present 3999 >> Node 0, zone DMA32 >> spanned 1044480 >> present 439600 > > I suspect this one is contigous ;-) Just checked. It's not. Probably because there are some holes that are entirely without a memmap. (PCI hole) -- Cheers, David