linux-kernel - Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings

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Message-ID: <20230502112355.x5lgcy75lh4mjo66@quack3>
Date:   Tue, 2 May 2023 13:23:55 +0200
From:   Jan Kara <jack@...e.cz>
To:     Peter Zijlstra <peterz@...radead.org>
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>,
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        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>,
        David Hildenbrand <david@...hat.com>,
        Dave Chinner <david@...morbit.com>,
        Theodore Ts'o <tytso@....edu>, Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing
 to file-backed mappings

On Tue 02-05-23 13:13:34, Peter Zijlstra wrote:
> On Tue, May 02, 2023 at 12:11:49AM +0100, Lorenzo Stoakes wrote:
> > +
> > +		/*
> > +		 * Neither anonymous nor shmem-backed folios require
> > +		 * dirty tracking.
> > +		 */
> > +		ret = folio_test_anon(folio) ||
> > +			(mapping && shmem_mapping(mapping));
> > +	} else {
> > +		/* If the mapping is unstable, fallback to the slow path. */
> > +		ret = false;
> > +	}
> > +
> > +	unlock_rcu();
> > +
> > +	return ret;
> 
> then becomes:
> 
> 
> 	if (folio_test_anon(folio))
> 		return true;
> 
> 	/*
> 	 * 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;

Yeah, that's even better.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR