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 7E6FB39B4AF for ; Mon, 7 Sep 2026 14:27:55 +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=1788791277; cv=none; b=tpp48naDsEglyyqjUY152FcROQ33OhYT9MtEVOqwnmpBGzfNG48FsyCUPuvULeRB0cEWnuoValQST0nr4kbaIS/RB6z2/1FnF4T0Co4NYDk3oqZTA3q/yGkEOTv0qFmuTIzDVotHjyaA5x59JcTY1JI8xrHcxlHZ/AsyCIfsmlw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788791277; c=relaxed/simple; bh=pGRy+E82qR8jPuNhD4AJZnlqd9rOGEhFr+3llWnVY6o=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=pr7QkqU+BsNp4xdI4sKZUy9Llr7lw+68sqIRWM4msiKgfZbm74PpbnnmAUXhHgBwI7e7FX9rPPkh7oEmpKmK4S2V65uZOyl2eCrF0x/hThLKGPp2P2m0s+ToNT+MkodnD7Fft8HZ86Ffy/1rUemIXzbJjkIdXTgprZuxcSugMYM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=TectPzyv; 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="TectPzyv" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 102D81F00A3A; Mon, 7 Sep 2026 14:27:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788791275; bh=L2LUyi3JasTalKXaDzwg8qu2XnDVZGCw0xs9cKv1TcE=; h=Date:Subject:To:Cc:References:From:In-Reply-To; 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 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: 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; 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charset=UTF-8 Content-Transfer-Encoding: 7bit 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