fs: don't flush pagecache when expanding block device

When changing the size of a block device, its all caches are freed.
It's necessary on shrinking to prevent spurious I/Os to the disappeared
region.  However, on expanding, such kind of I/Os doesn't happen.

Similar things can be considered for btrfs filesystem resize and
resize2fs, but they are designed not to drop caches when expanding.
Therefore this patch removes unnecessary cache drop.

Link: http://lkml.kernel.org/r/1521457240-153390-1-git-send-email-shunki-fujita@cybozu.co.jp
Signed-off-by: Shunki Fujita <shunki-fujita@cybozu.co.jp>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/block_dev.c

@@ -1324,7 +1324,8 @@ static void flush_disk(struct block_device *bdev, bool kill_dirty)
  * @bdev: struct bdev to adjust.
  *
  * This routine checks to see if the bdev size does not match the disk size
- * and adjusts it if it differs.
+ * and adjusts it if it differs. When shrinking the bdev size, its all caches
+ * are freed.
  */
 void check_disk_size_change(struct gendisk *disk, struct block_device *bdev)
 {
@@ -1337,7 +1338,8 @@ void check_disk_size_change(struct gendisk *disk, struct block_device *bdev)
 		       "%s: detected capacity change from %lld to %lld\n",
 		       disk->disk_name, bdev_size, disk_size);
 		i_size_write(bdev->bd_inode, disk_size);
-		flush_disk(bdev, false);
+		if (bdev_size > disk_size)
+			flush_disk(bdev, false);
 	}
 }
 EXPORT_SYMBOL(check_disk_size_change);