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

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Date:   Tue, 25 Apr 2023 00:26:25 +0100
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Christoph Hellwig <hch@...radead.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        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>
Subject: Re: [PATCH v2] mm/gup: disallow GUP writing to file-backed mappings
 by default

On Mon, Apr 24, 2023 at 08:17:11PM -0300, Jason Gunthorpe wrote:
> On Tue, Apr 25, 2023 at 12:03:34AM +0100, Lorenzo Stoakes wrote:
>
> > Except you dirty a page that is mapped elsewhere that thought everything
> > was cleaned and... not sure the PTLs really help you much?
>
> If we have a writable PTE then while the PTE's PTL is held it is impossible
> for a FS to make the page clean as any cleaning action has to also
> take the PTL to make the PTE non-present or non-writable.
>

That's a very good point! Passing things back with a spinlock held feels
pretty icky though, and obviously a no-go for a FOLL_PIN. Perhaps for a
FOLL_GET this would be workable.

> > If we want to be more adventerous the opt-in variant could default to on
> > for FOLL_LONGTERM too, but that discussion can be had over on that patch
> > series.
>
> I think you should at least do this too to explain why io_uring code
> is moving into common code..
>

OK, I'll respin this as a v3 of this series then since we'll be defaulting
FOLL_LONGTERM at least (for which there seems to be broad consensus), but
also permit this flag to be set manually and set it for io_uring.

> Jason