netdev - Re: [PATCH v2] mm/gup: disallow GUP writing to file-backed mappings by default

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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.