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Message-ID: <90a54439-5d30-4711-8a86-eba816782a66@lucifer.local> Date: Mon, 24 Apr 2023 11:17:55 +0100 From: Lorenzo Stoakes <lstoakes@...il.com> To: Christoph Hellwig <hch@...radead.org> Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>, Jason Gunthorpe <jgg@...pe.ca>, Jens Axboe <axboe@...nel.dk>, Matthew Wilcox <willy@...radead.org>, Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>, Leon Romanovsky <leon@...nel.org>, Christian Benvenuti <benve@...co.com>, Nelson Escobar <neescoba@...co.com>, Bernard Metzler <bmt@...ich.ibm.com>, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, Arnaldo Carvalho de Melo <acme@...nel.org>, Mark Rutland <mark.rutland@....com>, Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>, Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>, Bjorn Topel <bjorn@...nel.org>, Magnus Karlsson <magnus.karlsson@...el.com>, Maciej Fijalkowski <maciej.fijalkowski@...el.com>, Jonathan Lemon <jonathan.lemon@...il.com>, "David S . Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Christian Brauner <brauner@...nel.org>, Richard Cochran <richardcochran@...il.com>, Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, Jesper Dangaard Brouer <hawk@...nel.org>, John Fastabend <john.fastabend@...il.com>, linux-fsdevel@...r.kernel.org, linux-perf-users@...r.kernel.org, netdev@...r.kernel.org, bpf@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>, Jason Gunthorpe <jgg@...dia.com> Subject: Re: [PATCH v2] mm/gup: disallow GUP writing to file-backed mappings by default On Mon, Apr 24, 2023 at 02:43:56AM -0700, Christoph Hellwig wrote: > I'm pretty sure DIRECT I/O reads that write into file backed mappings > are out there in the wild. > > So while I wish we had never allowed this, the exercise seems futile and > instead we need to work on supporting this usecase, with the FOLL_PIN > infrastructure being a big step toward that. It's not entirely futile, there's at least one specific use case, which is io_uring which is currently open coding an equivalent check themselves. By introducing this change we prevent them from having to do so and provide a means by which other callers who implicitly need this do not have to do so either. In addition, this change frees up a blocked patch series intending to clean up GUP which should help open the door to further improvements across the system. So I would argue certainly not futile. In addition, I think it's useful to explicitly document that this is a broken case and, through use of the flag, highlight places which are problematic (although perhaps not exhaustively). I know Jason is keen on fixing this at a fundamental level and this flag is ultimately his suggestion, so it certainly doesn't stand in the way of this work moving forward.
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