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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-438bd5732edsm24173295e9.36.2025.01.24.04.29.13 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 24 Jan 2025 04:29:13 -0800 (PST) Message-ID: <752b098c-345e-4374-bf01-37193b402890@redhat.com> Date: Fri, 24 Jan 2025 13:29:12 +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: Jeff Layton , Miklos Szeredi Cc: Shakeel Butt , Joanne Koong , Bernd Schubert , Zi Yan , 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: <791d4056-cac1-4477-a8e3-3a2392ed34db@redhat.com> <1fdc9d50-584c-45f4-9acd-3041d0b4b804@redhat.com> <54ebdef4205781d3351e4a38e5551046482dbba0.camel@kernel.org> <2848b566-3cae-4e89-916c-241508054402@redhat.com> <3c09175916c2a56b55d9cf61e4d8c0ea66156da2.camel@kernel.org> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Queue-Id: AFE8E1A0006 X-Rspamd-Server: rspam10 X-Stat-Signature: xwf5tojcqurgx5wj3eoukf8q63ic1uww X-HE-Tag: 1737721758-867570 X-HE-Meta: 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 t9Qj85/1 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 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 14.01.25 16:49, Jeff Layton wrote: > On Tue, 2025-01-14 at 10:40 +0100, Miklos Szeredi wrote: >> On Tue, 14 Jan 2025 at 09:38, Miklos Szeredi wrote: >> >>> Maybe an explicit callback from the migration code to the filesystem >>> would work. I.e. move the complexity of dealing with migration for >>> problematic filesystems (netfs/fuse) to the filesystem itself. I'm >>> not sure how this would actually look, as I'm unfamiliar with the >>> details of page migration, but I guess it shouldn't be too difficult >>> to implement for fuse at least. >> >> Thinking a bit... >> >> 1) reading pages >> >> Pages are allocated (PG_locked set, PG_uptodate cleared) and passed to >> ->readpages(), which may make the pages uptodate asynchronously. If a >> page is unlocked but not set uptodate, then caller is supposed to >> retry the reading, at least that's how I interpret >> filemap_get_pages(). This means that it's fine to migrate the page >> before it's actually filled with data, since the caller will retry. >> >> It also means that it would be sufficient to allocate the page itself >> just before filling it in, if there was a mechanism to keep track of >> these "not yet filled" pages. But that probably off topic. >> > > Sounds plausible. > >> 2) writing pages >> >> When the page isn't actually being copied, the writeback could be >> cancelled and the page redirtied. At which point it's fine to migrate >> it. The problem is with pages that are spliced from /dev/fuse and >> control over when it's being accessed is lost. Note: this is not >> actually done right now on cached pages, since writeback always copies >> to temp pages. So we can continue to do that when doing a splice and >> not risk any performance regressions. >> > > Can we just cancel and redirty the page like that when doing a > WB_SYNC_ALL flush? I think we'd need to ensure that it gets a new > writeback attempt as soon as the migration is done if that's in > progress, no? Yeah, that was one of my initial questions as well: could one "transparently" (to user space) handle canceling writeback and simply re-dirty the page. -- Cheers, David / dhildenb