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

From: Lorenzo Stoakes <lstoakes@gmail.com>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: Theodore Ts'o <tytso@mit.edu>,
	David Hildenbrand <david@redhat.com>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Jens Axboe <axboe@kernel.dk>,
	Matthew Wilcox <willy@infradead.org>,
	Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>,
	Leon Romanovsky <leon@kernel.org>,
	Christian Benvenuti <benve@cisco.com>,
	Nelson Escobar <neescoba@cisco.com>,
	Bernard Metzler <bmt@zurich.ibm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@kernel.org>, Namhyung Kim <namhyung@kernel.org>,
	Ian Rogers <irogers@google.com>,
	Adrian Hunter <adrian.hunter@intel.com>,
	Bjorn Topel <bjorn@kernel.org>,
	Magnus Karlsson <magnus.karlsson@intel.com>,
	Maciej Fijalkowski <maciej.fijalkowski@intel.com>,
	Jonathan Lemon <jonathan.lemon@gmail.com>,
	"David S . Miller" <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	Christian Brauner <brauner@kernel.org>,
	Richard Cochran <richardcochran@gmail.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Jesper Dangaard Brouer <hawk@kernel.org>,
	John Fastabend <john.fastabend@gmail.com>,
	linux-fsdevel@vger.kernel.org, linux-perf-users@vger.kernel.org,
	netdev@vger.kernel.org, bpf@vger.kernel.org,
	Oleg Nesterov <oleg@redhat.com>,
	John Hubbard <jhubbard@nvidia.com>, Jan Kara <jack@suse.cz>,
	"Kirill A . Shutemov" <kirill@shutemov.name>,
	Pavel Begunkov <asml.silence@gmail.com>,
	Mika Penttila <mpenttil@redhat.com>,
	David Howells <dhowells@redhat.com>,
	Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings by default
Date: Sun, 30 Apr 2023 00:09:35 +0100	[thread overview]
Message-ID: <08ff7961-7e86-40b3-8e25-1592526c94d4@lucifer.local> (raw)
In-Reply-To: <ZE2ht9AGx321j0+s@nvidia.com>

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.
>
> If this upsets people they can work on fixing it, but at least we
> don't have these kernel problems and inconsistencies to deal with.
>

Indeed, I think there is a broad consensus that FOLL_LONGTERM is such an
egregious case that this is the way forward.

I will respin with the discussed GUP-fast changes relatively soon and then
we can see where that takes us :)

> Jason