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Date:   Tue, 2 May 2023 14:40:58 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Lorenzo Stoakes <lstoakes@gmail.com>
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Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing
 to file-backed mappings
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References: <cover.1682981880.git.lstoakes@gmail.com>
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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().