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

From: Jens Axboe <axboe@kernel.dk>
To: David Hildenbrand <david@redhat.com>,
	Lorenzo Stoakes <lstoakes@gmail.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Jason Gunthorpe <jgg@ziepe.ca>,
	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>, Jason Gunthorpe <jgg@nvidia.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: Fri, 28 Apr 2023 09:15:08 -0600	[thread overview]
Message-ID: <447d0270-9c0e-23f4-3c62-33c3eff325af@kernel.dk> (raw)
In-Reply-To: <a8561203-a4f3-4b3d-338a-06a60541bd6b@redhat.com>

On 4/28/23 9:13?AM, David Hildenbrand wrote:
>>> I know, Jason und John will disagree, but I don't think we want to be very
>>> careful with changing the default.
>>>
>>> Sure, we could warn, or convert individual users using a flag (io_uring).
>>> But maybe we should invest more energy on a fix?
>>
>> This is proactively blocking a cleanup (eliminating vmas) that I believe
>> will be useful in moving things forward. I am not against an opt-in option
>> (I have been responding to community feedback in adapting my approach),
>> which is the way I implemented it all the way back then :)
> 
> There are alternatives: just use a flag as Jason initially suggested
> and use that in io_uring code. Then, you can also bail out on the
> GUP-fast path as "cannot support it right now, never do GUP-fast".

Since I've seen this brougth up a few times, what's the issue on the
io_uring side? We already dropped the special vma checking, it's in -git
right. Hence I don't believe there are any special cases left for
io_uring at all, and we certainly don't allow real file backings either,
never have done.

-- 
Jens Axboe