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: <20230502124058.GB1597602@hirez.programming.kicks-ass.net>
Date:   Tue, 2 May 2023 14:40:58 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Lorenzo Stoakes <lstoakes@...il.com>
Cc:     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>,
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        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>,
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        Paul McKenney <paulmck@...nel.org>
Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing
 to file-backed mappings

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().