[<prev] [next>] [<thread-prev] [day] [month] [year] [list]
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().
Powered by blists - more mailing lists