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[2003:cb:c707:5e00:9e97:86d:5ed5:bb95]) by smtp.gmail.com with ESMTPSA id v21-20020a05600c429500b003dc1a525f22sm2429845wmc.25.2023.01.26.15.41.09 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 26 Jan 2023 15:41:09 -0800 (PST) Message-ID: <5a1796f3-e49c-80e8-2dd6-9a6e82939271@redhat.com> Date: Fri, 27 Jan 2023 00:41:08 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.6.0 Subject: Re: [PATCH v7 2/8] iov_iter: Add a function to extract a page list from an iterator To: Al Viro Cc: David Howells , Christoph Hellwig , Matthew Wilcox , Jens Axboe , Jan Kara , Jeff Layton , Logan Gunthorpe , linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, Christoph Hellwig , John Hubbard , linux-mm@kvack.org References: <7bbcccc9-6ebf-ffab-7425-2a12f217ba15@redhat.com> <246ba813-698b-8696-7f4d-400034a3380b@redhat.com> <20230120175556.3556978-1-dhowells@redhat.com> <20230120175556.3556978-3-dhowells@redhat.com> <3814749.1674474663@warthog.procyon.org.uk> <3903251.1674479992@warthog.procyon.org.uk> From: David Hildenbrand Organization: Red Hat In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: DA11D1C0011 X-Rspam-User: X-Stat-Signature: 93jqcfet9rsnnkao4bpkde69dutgqehr X-HE-Tag: 1674776473-825600 X-HE-Meta: U2FsdGVkX1/kEr8weawQqGqLAv+bHLHG1hPjgiG47zu0AOI6OIXgkerVgVPoS/xeBZvAvyRiIR6NaRLeCIn5gA7BmmLC/a7hRxl+Liv+MPNpE+zZf3gz3YSi1InquJZoOhaeldWzOxO4WzPVOpRddOwjqHK01jQpGT768WUdkkcvbiq4k/BmTnUjh4B+PWY9bqU68RGORhOHR5om8hyU00DLG0grEgCKXyyEf3uODlete7g1m+kWSfyM4Mnsz5LB7vy2zwzIHszu29egTCxqMCLCEf1n8vuyu6vIz9xqmb6MoBXv4nPmeMquZnLNbBzYngQdGY6fvygSFCNOqKAs/uuFCxQvcdb043LTO6udEmeufquD+Lz+otAweNFXysgnnS2rNKs+vnWTbFc3PBgySq15bc7u7XGSjw8w7q35uHL7Yx1D+qsWxDFXskWEb9dqecBwY1Y8zf4IIgDqDHa2lh/GTk+Geio80e85zQBsJO3Kli4Qc0mgvo5VpCLKQeUAUdBwEPfs7LB4LMt9qFnYzQA+rKURDCQauDj4+jqpr2PkLx6SHWeH36nHSMIDSidoTjuTMXF2TUM3fcj38+g9f1i1gcgeO0fNjmtmqB5jVeG5gzdHvFFfCQCVinJP9NB2C6bkQ8ipSGO6pVOCHrbPZwqlv2s3mUyamrrdo27WBccQnX9Qpe4jcE/+4QUH1glxmAz5G5k7VlUXlzsQPE1lHacy30tad4hOZhow0SOjoPkD6frE67kUcMuWecgV1nEXb6X8tXCRTMUvUP2cMXBENfNPrE+gKf1legDLzR5EdoSW8aBfjfKxrjylPGbGcHXFKLjvzTlKLRnFIKJCvxpakszLsKW+vM9KgcNeNQyf022jOcTC4vvdj8fkJkixmrQbWwerAIDPJ4Nlh6IFwyH1f6P/l1R1RQTjD/W6WHsYXo/L/98qQICN6w02FJOxYq9aIRsj4nC6rt8ymO4BeRQ cgX0+TXF RoLVxmwMcSLhqVeOaL2vzPwoKM8TKlDO1qtOJ1l9PgJPZI4GWyJhoCbuXRcelhs6MOfo0UeJzre5Oo0gsbyrHzacHzg/XHM1yMRC/PpceH7T9Be9m4l/uOwdRnIzE38tzb42Qd1S4auh9HSafiK47jmflQQIx6rFllfQw26ga0uzY0rTQFcGdaBen8dffeG0+AslrHk+TtXmQv5+CHU+io3c6URnZKlbXm9cFSa1IB4HgdP9O2dGwj/htI9xSZIkGi4FoDRjGn7VzKwu2v797RDCdFAHjxIxvoZ1eci9F5AaQ4RIrIbHX6bIrtaRZ1RexyV15xJcCznMImQ8ADxKsCiMHYCXYcm6v2FhORGLgH1HNwPzw6uPProPYCQn+IhbGukV7qds6s82B1nQ7d9WHtdy9QaR5VK+bUrMpj7ScEiGcBGHdisn4EW9W3U1cIY6u6YwOIDIHe5ViNzRMDLRL8Ggm2BVACkXi24qK0fEkhumGL61JQ2xHjJhaLd/ZZVYFOo7Nl5uuuF1gkSNRs8lW0CmyWxEgn2lvuh/S 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: On 26.01.23 23:15, Al Viro wrote: > On Mon, Jan 23, 2023 at 02:24:13PM +0100, David Hildenbrand wrote: >> On 23.01.23 14:19, David Howells wrote: >>> David Hildenbrand wrote: >>> >>>> Switching from FOLL_GET to FOLL_PIN was in the works by John H. Not sure what >>>> the status is. Interestingly, Documentation/core-api/pin_user_pages.rst >>>> already documents that "CASE 1: Direct IO (DIO)" uses FOLL_PIN ... which does, >>>> unfortunately, no reflect reality yet. >>> >>> Yeah - I just came across that. >>> >>> Should iov_iter.c then switch entirely to using pin_user_pages(), rather than >>> get_user_pages()? In which case my patches only need keep track of >>> pinned/not-pinned and never "got". >> >> That would be the ideal case: whenever intending to access page content, use >> FOLL_PIN instead of FOLL_GET. >> >> The issue that John was trying to sort out was that there are plenty of >> callsites that do a simple put_page() instead of calling unpin_user_page(). >> IIRC, handling that correctly in existing code -- what was pinned must be >> released via unpin_user_page() -- was the biggest workitem. >> >> Not sure how that relates to your work here (that's why I was asking): if >> you could avoid FOLL_GET, that would be great :) > > Take a good look at iter_to_pipe(). It does *not* need to pin anything > (we have an ITER_SOURCE there); with this approach it will. And it > will stuff those pinned references into a pipe, where they can sit > indefinitely. > > IOW, I don't believe it's a usable approach. > Not sure what makes you believe that FOLL_GET is any better for this long-term pinning, I'd like to learn about that. As raised already somewhere in the whole discussion by me, the right way to take such a long-term ping as vmsplice() does is to use FOLL_PIN|FOLL_LONGTERM. As also raised, that will fix the last remaining vmsplice()+hugetlb COW issue as tested by the cow.c vm selftest and make sure to migrate that memory off of MIGRATE_MOVABLE/CMA memory where we cannot tolerate to have long-term unmovable memory sitting around. -- Thanks, David / dhildenb