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Date:   Tue, 2 May 2023 13:23:55 +0200
From:   Jan Kara <jack@suse.cz>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Lorenzo Stoakes <lstoakes@gmail.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
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Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing
 to file-backed mappings
Message-ID: <20230502112355.x5lgcy75lh4mjo66@quack3>
References: <cover.1682981880.git.lstoakes@gmail.com>
 <dee4f4ad6532b0f94d073da263526de334d5d7e0.1682981880.git.lstoakes@gmail.com>
 <20230502111334.GP1597476@hirez.programming.kicks-ass.net>
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In-Reply-To: <20230502111334.GP1597476@hirez.programming.kicks-ass.net>
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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@suse.com>
SUSE Labs, CR