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

From: John Hubbard <jhubbard@nvidia.com>
To: David Hildenbrand <david@redhat.com>,
	Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Peter Xu <peterx@redhat.com>,
	"Kirill A . Shutemov" <kirill@shutemov.name>,
	Jason Gunthorpe <jgg@nvidia.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>, Jan Kara <jack@suse.cz>,
	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 11:26:59 -0700	[thread overview]
Message-ID: <ee1426d4-062d-74d5-8a9d-8bb06d554518@nvidia.com> (raw)
In-Reply-To: <7e096879-4578-36df-4809-3b04f4c20587@redhat.com>

On 4/28/23 10:29, David Hildenbrand wrote:
...
>> Is that an issue with interrupts disabled though? Will block page tables being
>> removed and as Kirill says (sorry I maybe misinterpreted you) we should be ok.
> 
> Let's rule out page table freeing. If our VMA only spans a single page 
> and falls into the same PMD as another VMA, an munmap() would not even 
> free a single page table.
> 
> However, if unmapping a page (flushing the TLB) would imply an IPI as 
> Kirill said, we'd be fine. I recall that that's not the case for all 
> architectures, but I might be just wrong.
> 

Some architectures use IPIs, and others use an RCU-like approach instead.

thanks,
-- 
John Hubbard
NVIDIA