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 718C722EE5 for ; Fri, 3 Oct 2025 01:02:37 +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=1759453369; cv=none; b=pzNst4LYMq8b/QG88Zjxpi9UCBTgZQ3s3IIREIukLEmhOi5yHawQx8+eE8dwEMyueouBEBOg/WBKswk/Pu5FYLpkqUYSmR4INr5BpVjdwKVPfU8ZcvennXVhe4lFrKnFEhpdQNqYuOaUuGIrJofVr5HRXCoA5+EDb0RfA1xqtgU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759453369; c=relaxed/simple; bh=V3N6JPVTAJbFIPW0aA6Vkefg9rVrdmnnmo+rjMA5qWY=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=UQEdGeizxAkc3VI6Q6PaNDGcDlVXyvKqMLX47qBUO2Fi7gISKALkizZlik/CGn0t87g8NZPxJMIJTf58fVUwVhZMtYgYcZjOE7OIPXi4jjsBouCNESBdAdnCjkx3OWzV2MD7ZhPEFoRhxpE/3IgGGrpMMxPg3I6IQ11F8Rkuhzo= 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-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> 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+NgFjrIIsWRmVeSWpSXmKPExsXC9ZZnoe5qhfsZBrcfKFnMWb+GzWLXjRCL 9Y3r2C2+rv/FbPHz7nF2i4uv/zBZ3F/2jMXi+NZ57BbX1x5lsri8aw6bxb01/1ktvvVJW1yY 2MtqceRNN7PF7x9A8blfDC1Wr8mw+LJ6FZvF7KP32B2EPdbMW8PosXPWXXaPPRNPsnl0t11m 99i8Qstj8Z6XTB6bPk1i91j4+wWzx4kZv1k8dj609Ohtfsfm8fHpLRaP9/uusnncvQ5Udu5a H7PH501yAQJRXDYpqTmZZalF+nYJXBmTl31hLdgrUDFv/XX2BsbfXF2MnBwSAiYSJ++9YO1i 5ACzP0yzAjFZBFQkXi73AKlgE1CXuHHjJzOILSKgIbGpbQOQzcXBLPCOWeLoilUsIAlhgXCJ 6e3nwGxeAQuJMwsusYLYQgKZEqc7ZjBBxAUlTs58AlbDLKAlcePfSyaQXcwC0hLL/3GAhDkF 7CSmf/oKtktUQFniwLbjTBBXnmKXOPmaDcKWlDi44gbLBEaBWUimzkIydRbC1AWMzKsYhTLz ynITM3NM9DIq8zIr9JLzczcxAiN2We2f6B2Mny4EH2IU4GBU4uH1KLiXIcSaWFZcmXuIUYKD WUmEN2HFnQwh3pTEyqrUovz4otKc1OJDjNIcLErivEbfylOEBNITS1KzU1MLUotgskwcnFIN jF43p3C5u1+dvMFPL8XH816PKa/ysydZ8ZZWZjPZTlgv2SxqYTVDaKWjerX5rA+MUpk3A94t N2J0Wqjyg5c/b9oEse+lstoLlLTeM4T4ucx529iwds7ixM+SJXW3mQwqL26uaHU48Pm0xA2X /vqWeXk7rt670//RSq4sZcH+U/PncHI9VhTcoMRSnJFoqMVcVJwIAChf3JzUAgAA X-Brightmail-Tracker: H4sIAAAAAAAAA+NgFlrJIsWRmVeSWpSXmKPExsXC5WfdrLta4X6Gwbl9EhZz1q9hs9h1I8Ri feM6douv638xW/y8e5zd4uLrP0wW95c9Y7E4vnUeu8XhuSdZLa6vPcpkcXnXHDaLe2v+s1p8 65O2uDCxl9Xi0LXnrBZH3nQzW/z+AZSc+8XQYvWaDIsvq1exWcw+eo/dQdRjzbw1jB47Z91l 99gz8SSbR3fbZXaPzSu0PBbvecnksenTJHaPhb9fMHucmPGbxWPnQ0uP3uZ3bB4fn95i8Xi/ 7yqbx93rQGXfbnt4LH7xgSlAKIrLJiU1J7MstUjfLoErY/KyL6wFewUq5q2/zt7A+Juri5GD Q0LAROLDNCsQk0VAReLlco8uRk4ONgF1iRs3fjKD2CICGhKb2jYA2VwczALvmCWOrljFApIQ FgiXmN5+DszmFbCQOLPgEiuILSSQKXG6YwYTRFxQ4uTMJ2A1zAJaEjf+vWQC2cUsIC2x/B8H SJhTwE5i+qevYLtEBZQlDmw7zjSBkXcWku5ZSLpnIXQvYGRexSiSmVeWm5iZY6pXnJ1RmZdZ oZecn7uJERiBy2r/TNzB+OWy+yFGAQ5GJR5ej4J7GUKsiWXFlbmHGCU4mJVEeBNW3MkQ4k1J rKxKLcqPLyrNSS0+xCjNwaIkzusVnpogJJCeWJKanZpakFoEk2Xi4JRqYCzJfXo+Qd+W68oF OUtTRkXL5W+uqspK5ZsXV1m0nbbiim9JOr/z46ZHcmEbjktPORXE8G0iZ3f3jfU/ODaUKhfe Wb02drl+/kaF09x+qu9ZEvPec28tSj7QZbLK+3PHSwPh+vWH7SMFLhavLObaktz/ydFeiusA s3t6TUQ+w+oXkiov94hyKLEUZyQaajEXFScCAH4UmxC8AgAA 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? > > > 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 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 >