From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from invmail4.hynix.com (exvmail4.hynix.com [166.125.252.92]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 5D50B2D3A9D for ; Fri, 3 Oct 2025 02:31:22 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=166.125.252.92 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759458694; cv=none; b=g+8Ik5CvLM4M7JxF0r4p2pndowIpRlZy0FENt3sGaj78VWtPh25xwVkcMcz3nyUTfzEPqFR18uEIwbrgrntodjTSXW2pIErwj+iVUvllkA6gpEg/DrSZjVxA/qGsRlUGuDkNxU2w+SsBdGv5LGV/zpXzTlt8tN4vr9SVfTh39jI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759458694; c=relaxed/simple; bh=9MqdsOYnzNI6LbgpUVMyBmakykkewjahkpVUC9MdEj8=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=sTFDMFpS7HrexVPNJcha8BRB5qmtETj19EPxZw9biqNNfkQ5t8Erw6HCUKcJ4H2NU6y8q9sHoprxgcEVCkMZq9WPtg6rZybegsIuglP/koQLZgQnoP7IVhTDvN3IZxjn6nP10QTcZVnUe0In5sc2AON16a66KBx886/7nYbwRCc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=sk.com; spf=pass smtp.mailfrom=sk.com; arc=none smtp.client-ip=166.125.252.92 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=sk.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=sk.com X-AuditID: a67dfc5b-c2dff70000001609-f1-68df3579b3c0 Date: Fri, 3 Oct 2025 11:31:16 +0900 From: Byungchul Park To: David Hildenbrand Cc: akpm@linux-foundation.org, ziy@nvidia.com, matthew.brost@intel.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, clameter@sgi.com, kravetz@us.ibm.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, max.byungchul.park@gmail.com, kernel_team@skhynix.com, harry.yoo@oracle.com, gwan-gyeong.mun@intel.com, yeoreum.yun@arm.com, syzkaller@googlegroups.com, ysk@kzalloc.com, Matthew Wilcox Subject: Re: [RFC] mm/migrate: make sure folio_unlock() before folio_wait_writeback() Message-ID: <20251003023116.GB29748@system.software.com> References: <20251002081612.53281-1-byungchul@sk.com> <9a586b5b-c47f-45eb-83c8-1e86431fc83d@redhat.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <9a586b5b-c47f-45eb-83c8-1e86431fc83d@redhat.com> User-Agent: Mutt/1.9.4 (2018-02-28) X-Brightmail-Tracker: H4sIAAAAAAAAA+NgFjrCIsWRmVeSWpSXmKPExsXC9ZZnoW6l6f0Mg1OLLCzmrF/DZrHrRojF +sZ17BZf1/9itvh59zi7xcXXf5gs7i97xmJxfOs8dovra48yWVzeNYfN4t6a/6wW3/qkLS5M 7GW1OPKmm9ni9w+g+Nwvhhar12RYfFm9is1i9tF77A7CHmvmrWH02DnrLrvHnokn2Ty62y6z e2xeoeWxeM9LJo9Nnyaxeyz8/YLZ48SM3yweOx9aevQ2v2Pz+Pj0FovH+31X2TzuXgcqO3et j9nj8ya5AIEoLpuU1JzMstQifbsEroyDb1+zFNwVrDjycRlTA2MvTxcjJ4eEgInEydVtzDD2 mYdnWEBsFgEViftt+xhBbDYBdYkbN36C1YgIaEhsatsAZHNxMAu8Y5Y4umIVWIOwQLjE9PZz YDavgIVE+9Q/YLaQQKbE6Y4ZTBBxQYmTM5+AxZkFtCRu/HsJFOcAsqUllv/jAAlzCthJTP/0 FWyXqICyxIFtx5lAdkkInGKXON48mxXiUEmJgytusExgFJiFZOwsJGNnIYxdwMi8ilEoM68s NzEzx0QvozIvs0IvOT93EyMwbpfV/onewfjpQvAhRgEORiUeXo+CexlCrIllxZW5hxglOJiV RHgTVtzJEOJNSaysSi3Kjy8qzUktPsQozcGiJM5r9K08RUggPbEkNTs1tSC1CCbLxMEp1cBo t5XpVJTr48tRjoLbam90xLE0M28937Rq/9bMFm9zHd9+9mvlWx6KV7D/2Jmg+MdsSpCCpl7c WXfR/4t1+38ZmfxZLrzFdkLMZdmACqFbNSf9PlxYKPHM2a2S6aNzT4+UduGN/+nc6dPWJr4P yVkkoXNDz1+yw9O+obHbd5Ghy67ZTzTuZSuxFGckGmoxFxUnAgD+MoZN1wIAAA== X-Brightmail-Tracker: H4sIAAAAAAAAA+NgFlrNIsWRmVeSWpSXmKPExsXC5WfdrFtpej/D4GanpsWc9WvYLHbdCLFY 37iO3eLr+l/MFj/vHme3uPj6D5PF/WXPWCyOb53HbnF47klWi+trjzJZXN41h83i3pr/rBbf +qQtLkzsZbU4dO05q8WRN93MFr9/ACXnfjG0WL0mw+LL6lVsFrOP3mN3EPVYM28No8fOWXfZ PfZMPMnm0d12md1j8wotj8V7XjJ5bPo0id1j4e8XzB4nZvxm8dj50NKjt/kdm8fHp7dYPN7v u8rmcfc6UNm32x4ei198YAoQiuKySUnNySxLLdK3S+DKOPj2NUvBXcGKIx+XMTUw9vJ0MXJy SAiYSJx5eIYFxGYRUJG437aPEcRmE1CXuHHjJzOILSKgIbGpbQOQzcXBLPCOWeLoilVgDcIC 4RLT28+B2bwCFhLtU/+A2UICmRKnO2YwQcQFJU7OfAIWZxbQkrjx7yVQnAPIlpZY/o8DJMwp YCcx/dNXsF2iAsoSB7YdZ5rAyDsLSfcsJN2zELoXMDKvYhTJzCvLTczMMdUrzs6ozMus0EvO z93ECIzCZbV/Ju5g/HLZ/RCjAAejEg+vR8G9DCHWxLLiytxDjBIczEoivAkr7mQI8aYkVlal FuXHF5XmpBYfYpTmYFES5/UKT00QEkhPLEnNTk0tSC2CyTJxcEo1MLZsuqQfb2GRoHvSu23p vy8RV+dpv3Zdc7JzXbGbsIHYwsDU1C9Pm5j36MeUhXtVHv24/fa2rQfuy/TvNWKfpf981rZH DR2b9KtU3q6R6LK+oymidfpYX779gvDw3I4p3q/7zVfHvGux/ekhePrThOnHn4ZviOhfXlXA e/PfHbnZsTFStyt/n1JiKc5INNRiLipOBAAgnFyIvgIAAA== X-CFilter-Loop: Reflected On Thu, Oct 02, 2025 at 01:38:59PM +0200, David Hildenbrand wrote: > > To simplify the scenario: > > > > Just curious, where is the __folio_start_writeback() to complete the > picture? ext4_end_io_end() was running as a wq worker after the io completion. DEPT report can tell that the following scenario happened with __folio_start_writeback() called far earlier, at least, before folio_test_writeback() was seen as true, but unfortunately DEPT doesn't capture the exact location of __folio_start_writeback(). Byungchul > > context X (wq worker) context Y (process context) > > > > migrate_pages_batch() > > ext4_end_io_end() ... > > ... migrate_folio_unmap() > > ext4_get_inode_loc() ... > > ... folio_lock() // hold the folio lock > > bdev_getblk() ... > > ... folio_wait_writeback() // wait forever > > __find_get_block_slow() > > ... ... > > folio_lock() // wait forever > > folio_unlock() migrate_folio_undo_src() > > ... > > ... folio_unlock() // never reachable > > ext4_finish_bio() > > ... > > folio_end_writeback() // never reachable > > > > But aren't you implying that it should from this point on be disallowed > to call folio_wait_writeback() with the folio lock held? That sounds ... > a bit wrong. > > Note that it is currently explicitly allowed: folio_wait_writeback() > documents "If the folio is not locked, writeback may start again after > writeback has finished.". So there is no way to prevent writeback from > immediately starting again. > > In particular, wouldn't we have to fixup other callsites to make this > consistent and then VM_WARN_ON_ONCE() assert that in folio_wait_writeback()? > > Of course, as we've never seen this deadlock before in practice, I do > wonder if something else prevents it? > > If it's a real issue, I wonder if a trylock on the writeback path could > be an option. > > -- > Cheers > > David / dhildenb >