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d=hostedemail.com; t=1759453357; a=rsa-sha256; cv=none; b=TkGVNmlhfiS3kBe5VU4gxfqTC7lGgYwg1MdIphCkdlIFNTGIQCHuFVhNxM3f0fvcLJzAIJ k856lWUgNc0lzyNntMVzpbBZbIbHve3grOUIQq5XuA2sZmAl7jnOk9ffVcj/VAF8FXSyfp 1shsWJr5HeQRpla7VN5IW/eF4+fVmLQ= ARC-Authentication-Results: i=1; imf27.hostedemail.com; dkim=none; dmarc=none; spf=pass (imf27.hostedemail.com: domain of byungchul@sk.com designates 166.125.252.92 as permitted sender) smtp.mailfrom=byungchul@sk.com X-AuditID: a67dfc5b-c2dff70000001609-fd-68df20ab7c6a Date: Fri, 3 Oct 2025 10:02:30 +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: <20251003010230.GC75385@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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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 Thank you for the information. I was wrong then. That means we shouldn't allow folio_lock() but try lock, while heading for writeback end in terms of dependency. Byungchul > 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 >