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b=TectPzyvQ6KaDruD02EunnARhdyeFcyCcgR0mB5w/mMMuJgYIDGR8EUcfGJDotTl4 goRR5661jo9SMB51F3ToVh3646QGVlKdTAi9vFGW6VuFzyeOrMs1plVFqpVs63PHNE jAm2jma2TATMCOIYXK/Q30IGd++zn5lkrsQA5CEBzum3P1oXRsTcxAa8ka6uNGJD7+ 2xq6mcnWfg7GgI2zefOyPSiztwtPRBA2cnb0ILaTqq2JfczSK/mIeqDixjLZiKEFW6 kIqrmFqQTT5+NwsHLsRjL8qiE6nyU49r0rt+FOZ0/W9LRaLbRXra2NhOgi2Y6/i7zR A1ui23uWP0Y+A== Message-ID: <1d625a60-9473-4f48-a384-7d756ccb7135@kernel.org> Date: Mon, 7 Sep 2026 16:27:50 +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: Yuan Liu , Oscar Salvador , Mike Rapoport , Wei Yang Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Chen Yu , Jason Zeng , Vlastimil Babka References: <20260903095543.3752338-1-yuan1.liu@intel.com> 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: <20260903095543.3752338-1-yuan1.liu@intel.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: B2A87180004 X-Stat-Signature: 3r7ffdicmdouem8p9jnzq4oocuibzpt9 X-Rspam-User: X-HE-Tag: 1788791276-399741 X-HE-Meta: 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 adCHl6Fk ysmupk1HhcHhwnQIJTDoxvDtu6Oh5qrDSgrT7cWPJmD2e9zUlT3xVthmM3yGjQ1O54GI1o89br3cjVBZT0330i5VP6uTPNMmztIJu3gsAmxnTnfPGXvi5UMF5ih4BnwmVrDr8Q7CiUlRLak7MyRZ0ws0S1phBJgzTrit8jFdNZQNwvHbSkuYMPJ2VJs1CvRY9S0gG72vUclNsRNq3pJV1uqM05wLErzpIRmvvEKOPK/kdT2lgTGyJlLLqUl7gigJ92IjhCqMUNfC1n60RXijOJZOItrRyUAAg74Sm5GcYFoiu9J27VGoc2AP+ywc9NfZ6rDzVgDAtgQExVw6qUXpt+JqDEqcRPM8E2WiCGVak6hn2py8fOs6wveirjMwn0yHFXz+MUbTcr+uz8Mx5+bYeGJuFjw== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 *** > If it can be hit with virtio-mem, it can be hit with any other memory hotunplug code path (e.g., dimm, dax). It is known that pfn_to_online_page() is racy. We usually expect the race window to be extremely small. But for compaction the race window is much larger. We do have get_online_mems()/put_online_mems(), big its the big hammer. We once discussed using RCU to protect pfn_to_online_page(), but I suspect for comapction that's not actually helpful (again, large race window). We'd have to use the memory notifier or a dedicated callback to let memory offlining sync with compaction. That's where it gets tricky :) It would be sufficient to let MEM_OFFLINE wait until any previous compaction users would be done with the range. In that case, the sections would be offline, but the memmap and zone range would not have been adjusted yet. -- Cheers, David