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From: Shakeel Butt To: David Hildenbrand Cc: Bernd Schubert , Joanne Koong , Zi Yan , miklos@szeredi.hu, linux-fsdevel@vger.kernel.org, jefflexu@linux.alibaba.com, josef@toxicpanda.com, linux-mm@kvack.org, kernel-team@meta.com, Matthew Wilcox , Oscar Salvador , Michal Hocko Subject: Re: [PATCH v6 4/5] mm/migrate: skip migrating folios under writeback with AS_WRITEBACK_INDETERMINATE mappings Message-ID: References: <968d3543-d8ac-4b5a-af8e-e6921311d5cf@redhat.com> <7b6b8143-d7a4-439f-ae35-a91055f9d62a@redhat.com> <2e13a67a-0bad-4795-9ac8-ee800b704cb6@fastmail.fm> <2bph7jx4hvhxpgp77shq2j7mo4xssobhqndw5v7hdvbn43jo2w@scqly5zby7bm> <71d7ac34-a5e5-4e59-802b-33d8a4256040@redhat.com> <9404aaa2-4fc2-4b8b-8f95-5604c54c162a@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <9404aaa2-4fc2-4b8b-8f95-5604c54c162a@redhat.com> X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 8EC6320036 X-Stat-Signature: yunhk6c5nyjrkhkyhe5zzwjnjr78cgzg X-HE-Tag: 1734717637-37553 X-HE-Meta: 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 xH5v0mpK KWIdcdtv+Psuafm2ywqgSVbcVDnX3Rs5eokS6/8U6zXBZ2eJV5QcwaakKMJxu+G36ASnyCrPRcMU8H4f6lj0xCxOo2L5LJZLmtvljmvtwMiuWHAeMjRNdTHeBKvDvMUnG6PAfKZsB7+gk2u1GdJ9f3q61bmdJhg1mFcWKrtwmeA77zueLrn+W4hu9OWYrApf34hvK5XFp9ZoLsqDda4n+eH76+NuiOcejcyqaNo5EBlJj6EDQMbzUIUdi8w== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Dec 20, 2024 at 03:49:39PM +0100, David Hildenbrand wrote: > > > I'm wondering if there would be a way to just "cancel" the writeback and > > > mark the folio dirty again. That way it could be migrated, but not > > > reclaimed. At least we could avoid the whole AS_WRITEBACK_INDETERMINATE > > > thing. > > > > > > > That is what I basically meant with short timeouts. Obviously it is not > > that simple to cancel the request and to retry - it would add in quite > > some complexity, if all the issues that arise can be solved at all. > > At least it would keep that out of core-mm. > > AS_WRITEBACK_INDETERMINATE really has weird smell to it ... we should try to > improve such scenarios, not acknowledge and integrate them, then work around > using timeouts that must be manually configured, and ca likely no be default > enabled because it could hurt reasonable use cases :( Just to be clear AS_WRITEBACK_INDETERMINATE is being used in two core-mm parts. First is reclaim and second is compaction/migration. For reclaim, it is a must have as explained by Jingbo in [1] i.e. due to potential self deadlock by fuse server. If I understand you correctly, the main concern you have is its usage in the second case. The reason for adding AS_WRITEBACK_INDETERMINATE in the second case was to avoid untrusted fuse server causing pain to unrelated jobs on the machine (fuse folks please correct me if I am wrong here). Now we are discussing how to better handle that scenario. I just wanted to point out that irrespective of that discussion, the reclaim will have handle the potential recursive deadlock and thus will be using AS_WRITEBACK_INDETERMINATE or something similar. [1] https://lore.kernel.org/all/d48ae58e-500f-4ef1-bc6f-a41a8f5b94bf@linux.alibaba.com/