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

[Jan Kara]

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@xxxxxxxx>
SUSE Labs, CR