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b=Q6sYONx4fuvB8ohfg8+Zb5w5d7/gn9W/4vtRWrOKMvagj3wgM/nawgz5XKxuLOl47 iZGjyhkFWYKjVZJ6lFjMW7296DNDcY8wHNOW3KoNLsfh6WMMN75TVbNdbjkntXgwTI mI8voiXMgefZKqwUzq3V4eeT/AgWaSGmS3dJSHVL4T82DdHDyVYG2vk9VvGom5KZCC nccaGG6IUSF3Go9qGmzcAibafzNrCrZpJlMlgH4rtsupW9b2fDHIniFxmQwjsgCOfk 6+z4lGZATU9iz90xg1cE/fKSCXNBAzIv8XVtBwdt5Rucbea4gKJxVaEqpMyUQI0upu +okyng6QED0ZQ== Message-ID: <12147238-575e-49ea-aa83-ea25c660ee38@kernel.org> Date: Thu, 10 Sep 2026 09:40:32 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [BUG] mm/memory_hotplug: panic due to race between compaction and memory hot-unplug To: Wei Yang Cc: Yuan Liu , Oscar Salvador , Mike Rapoport , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Chen Yu , Jason Zeng , Vlastimil Babka References: <20260903095543.3752338-1-yuan1.liu@intel.com> <1d625a60-9473-4f48-a384-7d756ccb7135@kernel.org> <20260910031609.qbn2qcxzbitau5xc@master> 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: <20260910031609.qbn2qcxzbitau5xc@master> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: bbsnat449u5sf1g9mu9wyixkyzp8z8c6 X-Rspamd-Queue-Id: B1C5E180005 X-Rspamd-Server: rspam07 X-HE-Tag: 1789026038-227515 X-HE-Meta: U2FsdGVkX18qsOWpt5eZ5Yc0CwmPB+auXvc5rcb8I0n/vFAEXhbDx1h4YEOt7kWatCMCDxm6NdtwV7HSl9wGuP4jKBGaB8bTqJ9GkQrXF3/i8INDZRlPMrGMuhFlcN8x2qnqA8xOkMRr+qZRyYvTwJhFFGU1fZAACidoQCvaO4gHB4rEj2MwF8k5K64nswA73UxKbmFlWIszs5ypS4dZ/lUhgatOEdX6B1y0BZ7sP0iMHPrgXZpf4R5ZS6qb7CsAm8ApumP+1LcufHRcgh4PHa1k8yBJXeJ68U/z0ObNKbtsldzbp0RHRWDsFqIqL5wyMmv1WPja/IUcpeiVK08rMLJBjYyd7pSeIIK/LaStnIV98KeDBQ89azcuMvhCqzGDYTvdEZeRnioZW/6b4zPa4jbi9RIbOWggWd4qRPahU0qqBDGPuWNPWps9M+MB3Uzjlbztva9h5fH1Di8soFNlLU9ThUx29y9oUSOuG0toEwhXYrsEB82DnyIn8S/uZgkPtSllLImbkSU3dnVil3bEOAjoUUj/QpWFblFoplglC/2OrcUvrvvBdNP0UpkjeavBzmW2PtZrmC3fj7akujO7aKYwIlgEUiZPD4mgTVPIZZ/UFJS15fY1d+WvyWp4tV6bioSH+oVZ/XLqmNTpQb6iV8G18nmpZ76wWufG4AkCeCckNz04dvQwGowA7I5uLsq7aehwPMBxbg6/67NVB34ZArS1l1f5cuBaVFfvC12wGwDCIX+EaFMsn1WtNmmKn0tAwIMYojmp3gLvdH/zqPrUIXaoB9DXbLSdJ44/TPfORbs72RcD1i9f4/Yo7bKTQGuAIkJxE1bE6KNQV9VwjvBuftWK+U/F8u8BKb7wJnt4qetkI0LzuXEHEXO5DYWsY2EaxV7Jc7bL7+7WURY9Pa68yyl0ODqLSIBKf3kE1aUTqBqYmh85Oht4kqb6MuAr3RuM57FE85pXTF8YLdfSqLF pKnl5v1t j7W5da7oOeJsRnqtAS9w37FKaSilVuJ7bshvGeiaub8PI1S8MhRct5ugpRJOieK0RYK3StefLtqy18xZqE+rUC0jxowpykKElUvRx/E5ux6C4RGPO5F8h9sxAsEBcvTSDUSJozY9uCNIGu6c1xd+edcWlqIrdsbULdLXxPbEDwx1ihz3hg933eTdzXddcjqzK+n0ot+VajuT/tNIDcG8Qz5HfXfqq1GTTN5D/zBZzWPeXFXouDmhkgMHHSrDa+DP2fFqLuOEXt2osAjWAdVQaQOzjtG9TGdVc6OVNRiA6KIFXPMa86HPDQyXqsNu8GE2AWc1vpFu3Un0L7Mr+PQZyEIS2uLu/2/sQfZWqXRxQmWDafOGe5rQIV+RFm7DnIWFdgBJG/Ub8667Q2L+4ziymb5BOKh3jnmYYrvkC Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/10/26 05:16, Wei Yang wrote: > On Mon, Sep 07, 2026 at 04:27:50PM +0200, David Hildenbrand (Arm) wrote: >> On 9/3/26 11:55, Yuan Liu wrote: >>> Hi all, >> >> Hi! >> >>> >>> While stress testing memory hotplug on a VM guest running an >>> unmodified vanilla mainline kernel (7.3.0-rc1, as reported by >>> uname -r), we hit a kernel panic in the guest when memory >>> hot-unplug runs concurrently with memory compaction. >>> >>> The kernel was built from mainline at commit: >>> >>> cee9395acd80 ("Linux 7.3-rc1") >>> >>> To be more specific, after a large virtio-mem hot-unplug, the guest >>> kernel takes a fatal page fault in suitable_migration_target(), called >>> from isolate_freepages() during compaction. >> >> Sounds like a real problem we should tackle. >> >>> >>> We are not sure whether this race is reachable under realistic >>> workloads or only under this synthetic stress test. Sharing it here >>> in case it is useful, and in case this is already a known issue. >>> Thanks. >>> >>> >>> Call trace (top to bottom) >>> ========================== >>> - RIP: suitable_migration_target+0x5/0x70 >>> isolate_freepages() <- compaction_alloc() <- >>> migrate_pages() <- compact_zone() <- compact_node() <- >>> sysctl_compaction_handler(). >>> >>> >>> Why the race happens >>> ==================== >>> CPU0 (compaction free-scanner) CPU1 (virtio-mem hot-unplug) >>> ---- ---- >>> page = pageblock_pfn_to_page() >>> /* checks pass, section ONLINE */ >>> /* returns valid struct page* */ >>> >>> offline_pages() >>> /* section -> offline */ >>> __remove_pages() >>> vmemmap_free() >>> /* struct page UNMAPPED */ >>> >>> suitable_migration_target(page) >>> PageBuddy(page) >>> read page->page_type >>> *** not-present fault -> panic *** >>> > > Does it mean, the combination of pfn_to_online_page() / PageBuddy(page) is not > safe, when there is hot-remove, even for other users. Not only compaction. There is an inherent race between any pfn_to_online_page() user that then takes a look at the memmap (PageBuddy, whatever). Usually, that's not really relevant, because for things to go terrible wrong (instead of only being slightly suboptimal :) ) You have to run both, memory offlining *and* memory removal. For things like page = pfn_to_online_page() if (PageBuddy(page)) That's unlikely to hit (no reports), and we could likely easily fix it with the help of RCU. It gets more problematic when we do things like page = pfn_to_online_page() ... do all other kind of stuff if (PageBuddy(page)) (what we have here) Using RCU is usually not easily possible. Mostly only comapction does something like that. -- Cheers, David