linux-kernel - Re: [PATCH v9 0/3] mm/gup: disallow GUP writing to file-backed mappings by default

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Date:   Wed, 17 May 2023 09:29:20 +0200
From:   Jan Kara <jack@...e.cz>
To:     Lorenzo Stoakes <lstoakes@...il.com>
Cc:     Jason Gunthorpe <jgg@...dia.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>, 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>,
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        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>,
        John Hubbard <jhubbard@...dia.com>, Jan Kara <jack@...e.cz>,
        Pavel Begunkov <asml.silence@...il.com>,
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        Theodore Ts'o <tytso@....edu>, Peter Xu <peterx@...hat.com>,
        Matthew Rosato <mjrosato@...ux.ibm.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>
Subject: Re: [PATCH v9 0/3] mm/gup: disallow GUP writing to file-backed
 mappings by default

On Mon 15-05-23 14:07:57, Lorenzo Stoakes wrote:
> On Mon, May 15, 2023 at 09:12:49AM -0300, Jason Gunthorpe wrote:
> > On Mon, May 15, 2023 at 12:16:21PM +0100, Lorenzo Stoakes wrote:
> > > Jason will have some thoughts on this I'm sure. I guess the key question
> > > here is - is it actually feasible for this to work at all? Once we
> > > establish that, the rest are details :)
> >
> > Surely it is, but like Ted said, the FS folks are not interested and
> > they are at least half the solution..
> 
> :'(

Well, I'd phrase this a bit differently - it is a difficult sell to fs
maintainers that they should significantly complicate writeback code / VFS
with bounce page handling etc. for a thing that is not much used corner
case. So if we can get away with forbiding long-term pins, then that's the
easiest solution. Dealing with short-term pins is easier as we can just
wait for unpinning which is implementable in a localized manner.

> > The FS also has to actively not write out the page while it cannot be
> > write protected unless it copies the data to a stable page. The block
> > stack needs the source data to be stable to do checksum/parity/etc
> > stuff. It is a complicated subject.
> 
> Yes my sense was that being able to write arbitrarily to these pages _at
> all_ was a big issue, not only the dirty tracking aspect.

Yes.

> I guess at some level letting filesystems have such total flexibility as to
> how they implement things leaves us in a difficult position.

I'm not sure what you mean by "total flexibility" here. In my opinion it is
also about how HW performs checksumming etc.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR