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[2003:cb:c70e:d000:4622:73e7:6184:8f0d]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-436724169afsm28333895e9.25.2024.12.21.08.25.36 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 21 Dec 2024 08:25:36 -0800 (PST) Message-ID: <61a4bcb1-8043-42b1-bf68-1792ee854f33@redhat.com> Date: Sat, 21 Dec 2024 17:25:35 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 4/5] mm/migrate: skip migrating folios under writeback with AS_WRITEBACK_INDETERMINATE mappings To: Joanne Koong Cc: Bernd Schubert , Shakeel Butt , 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 References: <43e13556-18a4-4250-b4fe-7ab736ceba7d@redhat.com> <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> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam05 X-Stat-Signature: c7bff53oopdgiik4x84undjmt36ucjqm X-Rspamd-Queue-Id: C54D240010 X-Rspam-User: X-HE-Tag: 1734798314-368737 X-HE-Meta: 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 Jh23iI4e xhzelmS1pgqYHvM5pp+Xjv2OaL7vMZL9IQdyf3lu1f37pogQnkVpeBp8Rtm7TwlWsd3EqxtfrT84uTZxgQp6VqO5G3A3YffN8dfyfsJ0GddFyDpKt543KFdhShGOYf4QR6AlopXFyzT78Pc9K5jkXRHWDhAOdpD0XUL68EpscPmffTymf2dhWICTBHslAQntLqP5E2S10WxZI9dcwfzw7NUJ66b6K5t+ZbIUMnnr1ghYUDlskNEENzE5RpO35iw+UMyvHEKlfcFP9NR9m4jwTTyTQaCySh1rs5nAjU1y9Itr31R7xmhjK8a6J1z7ryaLSTr1rSdQItGD7jMZIf4/ciCOHUKNhXPL11rDrC0xrR2BbHihYdlFSEPAS30horN1Mv1YsYaD+chMTUoy8SGpZ/abkhYBHUwGC4NHjTKJXThKWRoZbfTlY8xHXfvK3OD31TX/hpx3Rrzpo6aXL7BdiDJyP1mxiaEexGfHIqszfj0wBM/9Z3rZfmldqjg== X-Bogosity: Ham, tests=bogofilter, spamicity=0.007506, 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 20.12.24 22:01, Joanne Koong wrote: > On Fri, Dec 20, 2024 at 6:49 AM 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 :( >> >> Right now we clear the writeback flag immediately, indicating that data >> was written back, when in fact it was not written back at all. I suspect >> fsync() currently handles that manually already, to wait for any of the >> allocated pages to actually get written back by user space, so we have >> control over when something was *actually* written back. >> >> >> Similar to your proposal, I wonder if there could be a way to request >> fuse to "abort" a writeback request (instead of using fixed timeouts per >> request). Meaning, when we stumble over a folio that is under writeback >> on some paths, we would tell fuse to "end writeback now", or "end >> writeback now if it takes longer than X". Essentially hidden inside >> folio_wait_writeback(). >> >> When aborting a request, as I said, we would essentially "end writeback" >> and mark the folio as dirty again. The interesting thing is likely how >> to handle user space that wants to process this request right now (stuck >> in fuse_send_writepage() I assume?), correct? > > This would be fine if the writeback request hasn't been sent yet to > userspace but if it has and the pages are spliced Can you point me at the code where that splicing happens? , then ending > writeback could lead to memory crashes if the pipebuf buf->page is > accessed as it's being migrated. When a page/folio is being migrated, > is there some state set on the page to indicate that it's currently > under migration? Unfortunately not really. It should be isolated and locked. So it would be a !LRU but locked folio. -- Cheers, David / dhildenb