linux-kernel - Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings by default

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Message-ID: <20230501072718.GF2155823@dread.disaster.area>
Date:   Mon, 1 May 2023 17:27:18 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Theodore Ts'o <tytso@....edu>,
        David Hildenbrand <david@...hat.com>,
        Lorenzo Stoakes <lstoakes@...il.com>, 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>,
        John Hubbard <jhubbard@...dia.com>, Jan Kara <jack@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Pavel Begunkov <asml.silence@...il.com>,
        Mika Penttila <mpenttil@...hat.com>,
        David Howells <dhowells@...hat.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings
 by default

On Sat, Apr 29, 2023 at 08:01:11PM -0300, Jason Gunthorpe wrote:
> On Sat, Apr 29, 2023 at 12:21:09AM -0400, Theodore Ts'o wrote:
> 
> > In any case, the file system maintainers' position (mine and I doubt
> > Dave Chinner's position has changed) is that if you write to
> > file-backed mappings via GUP/RDMA/process_vm_writev, and it causes
> > silent data corruption, you get to keep both pieces, and don't go
> > looking for us for anything other than sympathy...
> 
> This alone is enough reason to block it. I'm tired of this round and
> round and I think we should just say enough, the mm will work to
> enforce this view point. Files can only be written through PTEs.

It has to be at least 5 years ago now that we were told that the
next-gen RDMA hardware would be able to trigger hardware page faults
when remote systems dirtied local pages.  This would enable
->page-mkwrite to be run on file backed pages mapped pages just like
local CPU write faults and everything would be fine.

Whatever happened to that? Are we still waiting for hardware that
can trigger page faults from remote DMA transfers, or have hardware
vendors given up on this?

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com