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Message-ID: <20230502083947.GE1597476@hirez.programming.kicks-ass.net> Date: Tue, 2 May 2023 10:39:47 +0200 From: Peter Zijlstra <peterz@...radead.org> To: Jan Kara <jack@...e.cz> Cc: "Kirill A . Shutemov" <kirill@...temov.name>, David Hildenbrand <david@...hat.com>, Peter Xu <peterx@...hat.com>, Lorenzo Stoakes <lstoakes@...il.com>, Jason Gunthorpe <jgg@...dia.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>, 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>, 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 Tue, May 02, 2023 at 10:00:16AM +0200, Jan Kara wrote: > On Sat 29-04-23 02:43:32, Kirill A . Shutemov wrote: > > I think I found relevant snippet of code that solves similar issue. > > get_futex_key() uses RCU to stabilize page->mapping after GUP_fast: > > > > > > /* > > * The associated futex object in this case is the inode and > > * the page->mapping must be traversed. Ordinarily this should > > * be stabilised under page lock but it's not strictly > > * necessary in this case as we just want to pin the inode, not > > * update the radix tree or anything like that. > > * > > * The RCU read lock is taken as the inode is finally freed > > * under RCU. If the mapping still matches expectations then the > > * mapping->host can be safely accessed as being a valid inode. > > */ > > rcu_read_lock(); > > > > if (READ_ONCE(page->mapping) != mapping) { > > rcu_read_unlock(); > > put_page(page); > > > > goto again; > > } > > > > inode = READ_ONCE(mapping->host); > > if (!inode) { > > rcu_read_unlock(); > > put_page(page); > > > > goto again; > > } > > > > I think something similar can be used inside GUP_fast too. > > Yeah, inodes (and thus struct address_space) is RCU protected these days so > grabbing RCU lock in gup_fast() will get you enough protection for checking > aops if you are careful (like the futex code is). GUP_fast has IRQs disabled per definition, there is no need to then also use rcu_read_lock().
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