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d=hostedemail.com; t=1759458684; a=rsa-sha256; cv=none; b=y92qDMqXOBuNZRBR3WsWGHp/JBABwJASswPkz6dlb9EuiGpDbUKXLxnXEiRDLW3iyb8YG9 e2L9rqNSN76YrStQqeI6vw17nnN93omRkt0Ofl94Irjh5Wa1zt2f8g34EYE53FaB0IAy6D s7Drc6orDq72bsHlN21A9DGC51XhJLQ= ARC-Authentication-Results: i=1; imf08.hostedemail.com; dkim=none; spf=pass (imf08.hostedemail.com: domain of byungchul@sk.com designates 166.125.252.92 as permitted sender) smtp.mailfrom=byungchul@sk.com; dmarc=none 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> MIME-Version: 1.0 Content-Type: text/plain; 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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 >