Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Tue, 2 May 2023 13:52:17 +0100
From:   Lorenzo Stoakes <lstoakes@gmail.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Peter Zijlstra <peterz@infradead.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Jason Gunthorpe <jgg@ziepe.ca>, 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>,
        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>,
        Jason Gunthorpe <jgg@nvidia.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>, Peter Xu <peterx@redhat.com>,
        Paul McKenney <paulmck@kernel.org>
Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing
 to file-backed mappings
Message-ID: <4529b057-19ae-408b-8433-7d220f1871c0@lucifer.local>
References: <cover.1682981880.git.lstoakes@gmail.com>
 <dee4f4ad6532b0f94d073da263526de334d5d7e0.1682981880.git.lstoakes@gmail.com>
 <20230502111334.GP1597476@hirez.programming.kicks-ass.net>
 <ab66d15a-acd0-4d9b-aa12-49cddd12c6a5@lucifer.local>
 <20230502120810.GD1597538@hirez.programming.kicks-ass.net>
 <20230502124058.GB1597602@hirez.programming.kicks-ass.net>
 <a597947b-6aba-bd8d-7a97-582fa7f88ad2@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <a597947b-6aba-bd8d-7a97-582fa7f88ad2@redhat.com>
Precedence: bulk

On Tue, May 02, 2023 at 02:47:22PM +0200, David Hildenbrand wrote:
> On 02.05.23 14:40, Peter Zijlstra wrote:
> > On Tue, May 02, 2023 at 02:08:10PM +0200, Peter Zijlstra wrote:
> >
> > > > >
> > > > >
> > > > > 	if (folio_test_anon(folio))
> > > > > 		return true;
> > > >
> > > > This relies on the mapping so belongs below the lockdep assert imo.
> > >
> > > Oh, right you are.
> > >
> > > > >
> > > > > 	/*
> > > > > 	 * Having IRQs disabled (as per GUP-fast) also inhibits RCU
> > > > > 	 * grace periods from making progress, IOW. they imply
> > > > > 	 * rcu_read_lock().
> > > > > 	 */
> > > > > 	lockdep_assert_irqs_disabled();
> > > > >
> > > > > 	/*
> > > > > 	 * Inodes and thus address_space are RCU freed and thus safe to
> > > > > 	 * access at this point.
> > > > > 	 */
> > > > > 	mapping = folio_mapping(folio);
> > > > > 	if (mapping && shmem_mapping(mapping))
> > > > > 		return true;
> > > > >
> > > > > 	return false;
> > > > >
> > > > > > +}
> >
> > So arguably you should do *one* READ_ONCE() load of mapping and
> > consistently use that, this means open-coding both folio_test_anon() and
> > folio_mapping().
>
> Open-coding folio_test_anon() should not be required. We only care about
> PAGE_MAPPING_FLAGS stored alongside folio->mapping, that will stick around
> until the anon page was freed.
>

Ack, good point!

> @Lorenzo, you might also want to special-case hugetlb directly using
> folio_test_hugetlb().
>

I already am :) I guess you mean when I respin along these lines? Will port
that across to.

> --
> Thanks,
>
> David / dhildenb
>