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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38c2a188fd0sm2597829f8f.58.2025.01.24.04.25.38 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 24 Jan 2025 04:25:39 -0800 (PST) Message-ID: <950a1ca6-0bf2-4aef-a6d4-b2a427c4ca54@redhat.com> Date: Fri, 24 Jan 2025 13:25:38 +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 , Miklos Szeredi Cc: Bernd Schubert , Jeff Layton , Shakeel Butt , 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> <060f4540-6790-4fe2-a4a5-f65693058ebf@fastmail.fm> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 83AE240011 X-Stat-Signature: bjiouc7k6ohzkiahdkwnfw9y4rr5nxeh X-HE-Tag: 1737721543-870668 X-HE-Meta: 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 6OYx1wWX 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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 21:51, Joanne Koong wrote: > On Tue, Jan 14, 2025 at 2:07 AM Miklos Szeredi wrote: >> >> On Tue, 14 Jan 2025 at 10:55, Bernd Schubert wrote: >>> >>> >>> >>> On 1/14/25 10:40, 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. >>> >>> With /dev/fuse buffer copies should be easy - just allocate the page >>> on buffer copy, control is in libfuse. >> >> I think the issue is with generic page cache code, which currently >> relies on the PG_locked flag on the allocated but not yet filled page. >> If the generic code would be able to keep track of "under >> construction" ranges without relying on an allocated page, then the >> filesystem could allocate the page just before copying the data, >> insert the page into the cache mark the relevant portion of the file >> uptodate. >> >>> With splice you really need >>> a page state. >> >> It's not possible to splice a not-uptodate page. >> >>> I wrote this before already - what is the advantage of a tmp page copy >>> over /dev/fuse buffer copy? I.e. I wonder if we need splice at all here. >> >> Splice seems a dead end, but we probably need to continue supporting >> it for a while for backward compatibility. > > For the splice case, could we do something like this or is this too invasive?: > * in mm, add a flag that marks a page as either being in migration or > temporarily blocking migration > * in splice, when we have to access the page in the pipe buffer, check > if that flag is set and wait for the migration to complete before > proceeding > * in splice, set that flag while it's accessing the page, which will > only temporarily block migration (eg for the duration of the memcpy) > > I guess this is basically what the page lock is for, but with less overhead? Yes, the folio lock kind-of behaves that way. One problem might be, that while the page is spliced that there is a raised refcount on the page: migration cannot make progress if there are unknown references. -- Cheers, David / dhildenb