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:

quoted

+
+		/*
+		 * 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 [off-list ref]
SUSE Labs, CR