mm/truncate: bail out early from invalidate_inode_pages2_range() if mapping is empty

If mapping is empty (both ->nrpages and ->nrexceptional is zero) we can
avoid pointless lookups in empty radix tree and bail out immediately
after cleancache invalidation.

Link: http://lkml.kernel.org/r/20170424164135.22350-4-aryabinin@virtuozzo.comSigned-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Alexey Kuznetsov <kuznet@virtuozzo.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Nikolay Borisov <n.borisov.lkml@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/truncate.c

@@ -624,6 +624,9 @@ int invalidate_inode_pages2_range(struct address_space *mapping,
 	int did_range_unmap = 0;
 
 	cleancache_invalidate_inode(mapping);
+	if (mapping->nrpages == 0 && mapping->nrexceptional == 0)
+		return 0;
+
 	pagevec_init(&pvec, 0);
 	index = start;
 	while (index <= end && pagevec_lookup_entries(&pvec, mapping, index,