From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 BC58135E936 for ; Thu, 10 Sep 2026 07:40:41 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789026047; cv=none; b=bic8AAwGD9KNtfHuN67NusyLJAftP6C8FiSE0XQ7ZQKqs06GG2BeLZeOnvubZSrnN/ajc0UphPCKZSFHggEOFMMgaJGGi0TT70ct7/8oTM6u1/XkmjERM8abee/96Rqojkvu6tCWUuyZZx/97z501GUaPqk/Dt9flGD7SZtnpzk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789026047; c=relaxed/simple; bh=u/w/W9pzN1EEBIsq7mUQ+ijV2/LPVyweohtt/TaTAHA=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=a3Y2gQTU8ZbddF7En1s4d8AfbeVYHpOReBZzOgUCyTvpfJA+qXrE+f0iqskLD25wdKGhBbuK2LT3UtpEhwhDBiI5qr3ApO0uGZpIwAmo3Mc9u8PqwdRB1ZmzFmE9AvhgyU+L8fQ3wxh0IHKhAkHBIVKH0FaCnIY3cFcRoOmnrLk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Q6sYONx4; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Q6sYONx4" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4A51F1F000FF; Thu, 10 Sep 2026 07:40:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789026037; bh=h9qej2utTGzUWAKjXHcQyamlOBo8eBDFXoNnSS57B6Q=; h=Date:Subject:To:Cc:References:From:In-Reply-To; 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 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: [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; 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charset=UTF-8 Content-Transfer-Encoding: 7bit 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