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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=VYe+QldW; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 08C66606A328 for ; Wed, 4 Nov 2026 09:08:29 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=VYe+QldW Received: by smtp.kernel.org (Postfix) with ESMTPSA id 162751F00A2B; Wed, 4 Nov 2026 09:08:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793783309; bh=+NYn9Sk4oUvq/RMmR/hLYOzDbKipm49pgybfm6AGhmA=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=VYe+QldWWMjvOsvqlc01rzx5B4pDG+f1Cm+nw8Pwbn8EK9cueIlJPWYrx+LxM3Gq/ gnnSF1Jl69zzE4NHq7IbZ4y3DIbwnSEtYVmg7P0Wip/ZRlqnVc3CEG4do9h6Fvw0UZ Uz48wStqcn5WtkMTzyATnh9Kj300OWhh+lyS2EkrP3Js1kWclwEy5loXPoOAG5y/jn WbmGxWwZ2gbuTBltl2OzRCY5uPgxjPaQfI4pZMNkckztmGtzcgtGijITFkzjmQWPsm xujdp7OtQSL2W7n+xzvlbHYcEef6D7aBzwP5PhP+885r9+F+ajwGNhCFoHTfeytHWH osB903NfBz1pA== Message-ID: <52a78290-3e4a-4a87-9bb5-5b0e80cf0be7@kernel.org> Date: Wed, 4 Nov 2026 10:08:23 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list 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: Ketan Kishore , 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> <12147238-575e-49ea-aa83-ea25c660ee38@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 11/4/26 06:18, Ketan Kishore wrote: > > > On 9/10/2026 1:10 PM, David Hildenbrand (Arm) wrote: >> On 9/10/26 05:16, Wei Yang wrote: >>> >>> 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. >> > > Hello > > We faced similar issue internally on one of our target's guest VM run, > where kcompactd de-referenced a vmemmap pointer for a PFN after the hot-unplug > path had already freed that vmemmap,with no lock preventing the two from running > simultaneously. > Call trace: >   suitable_migration_target+0x0/0x8c (P) >   migrate_pages_batch+0x1b4/0xc74 >   migrate_pages+0x150/0x654 >   compact_zone+0x5bc/0xb30 >   compact_node+0xa0/0x18c >   kcompactd+0x228/0x4d0 > > Would it be okay to introduce something like "try_get_online_mems" as shown > below to try a readlock in compaction path for hot-unplug to wait > for kcompactd to complete before freeing the vmemmap? Likely not what we want :) Gregory started working on some fixes: https://lore.kernel.org/r/20261027042204.3741307-1-gourry@gourry.net They need more discussions. They are a little bit low priority as we are fighting bigger fires ... -- Cheers, David