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Message-ID: <20230502172231.GH1597538@hirez.programming.kicks-ass.net> Date: Tue, 2 May 2023 19:22:32 +0200 From: Peter Zijlstra <peterz@...radead.org> To: David Hildenbrand <david@...hat.com> Cc: Lorenzo Stoakes <lstoakes@...il.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>, Jason Gunthorpe <jgg@...pe.ca>, 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>, Jason Gunthorpe <jgg@...dia.com>, John Hubbard <jhubbard@...dia.com>, Jan Kara <jack@...e.cz>, "Kirill A . Shutemov" <kirill@...temov.name>, Pavel Begunkov <asml.silence@...il.com>, Mika Penttila <mpenttil@...hat.com>, Dave Chinner <david@...morbit.com>, Theodore Ts'o <tytso@....edu>, Peter Xu <peterx@...hat.com>, Matthew Rosato <mjrosato@...ux.ibm.com>, "Paul E . McKenney" <paulmck@...nel.org>, Christian Borntraeger <borntraeger@...ux.ibm.com>, Mike Rapoport <rppt@...ux.ibm.com> Subject: Re: [PATCH v7 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings On Tue, May 02, 2023 at 07:13:49PM +0200, David Hildenbrand wrote: > [...] > > > +{ > > + struct address_space *mapping; > > + > > + /* > > + * GUP-fast disables IRQs - this prevents IPIs from causing page tables > > + * to disappear from under us, as well as preventing RCU grace periods > > + * from making progress (i.e. implying rcu_read_lock()). > > + * > > + * This means we can rely on the folio remaining stable for all > > + * architectures, both those that set CONFIG_MMU_GATHER_RCU_TABLE_FREE > > + * and those that do not. > > + * > > + * We get the added benefit that given inodes, and thus address_space, > > + * objects are RCU freed, we can rely on the mapping remaining stable > > + * here with no risk of a truncation or similar race. > > + */ > > + lockdep_assert_irqs_disabled(); > > + > > + /* > > + * If no mapping can be found, this implies an anonymous or otherwise > > + * non-file backed folio so in this instance we permit the pin. > > + * > > + * shmem and hugetlb mappings do not require dirty-tracking so we > > + * explicitly whitelist these. > > + * > > + * Other non dirty-tracked folios will be picked up on the slow path. > > + */ > > + mapping = folio_mapping(folio); > > + return !mapping || shmem_mapping(mapping) || folio_test_hugetlb(folio); > > "Folios in the swap cache return the swap mapping" -- you might disallow > pinning anonymous pages that are in the swap cache. > > I recall that there are corner cases where we can end up with an anon page > that's mapped writable but still in the swap cache ... so you'd fallback to > the GUP slow path (acceptable for these corner cases, I guess), however > especially the comment is a bit misleading then. > > So I'd suggest not dropping the folio_test_anon() check, or open-coding it > ... which will make this piece of code most certainly easier to get when > staring at folio_mapping(). Or to spell it out in the comment (usually I > prefer code over comments). So how stable is folio->mapping at this point? Can two subsequent reads get different values? (eg. an actual mapping and NULL) If so, folio_mapping() itself seems to be missing a READ_ONCE() to avoid the compiler from emitting the load multiple times.
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