Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings

From: Jason Gunthorpe <jgg@nvidia.com>
To: Peter Xu <peterx@redhat.com>
Cc: Matthew Rosato <mjrosato@linux.ibm.com>,
	David Hildenbrand <david@redhat.com>,
	Christian Borntraeger <borntraeger@linux.ibm.com>,
	Lorenzo Stoakes <lstoakes@gmail.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>,
	Dave Chinner <david@fromorbit.com>, Theodore Ts'o <tytso@mit.edu>
Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings
Date: Tue, 2 May 2023 12:36:30 -0300	[thread overview]
Message-ID: <ZFEt/ot6VKOgW1mT@nvidia.com> (raw)
In-Reply-To: <ZFEtKe/XcnC++ACZ@x1n>

On Tue, May 02, 2023 at 11:32:57AM -0400, Peter Xu wrote:
> > How does s390 avoid mmu notifiers without having lots of problems?? It
> > is not really optional to hook the invalidations if you need to build
> > a shadow page table..
> 
> Totally no idea on s390 details, but.. per my read above, if the firmware
> needs to make sure the page is always available (so no way to fault it in
> on demand), which means a longterm pinning seems appropriate here.
> 
> Then if pinned a must, there's no need for mmu notifiers (as the page will
> simply not be invalidated anyway)?

And what if someone deliberately changes the mapping?  memory hotplug
in the VM, or whatever?

We have ways to deal with this class of stuff in kvm and in
iommufd/vfio - this does not.

Jason