loop: use vfs_getattr_nosec for accurate file size

Use vfs_getattr_nosec() in lo_calculate_size() for getting the file
size, rather than just read the cached inode size via i_size_read().
This provides better results than cached inode data, particularly for
network filesystems where metadata may be stale.

Signed-off-by: Rajeev Mishra <[email protected]>
Reviewed-by: Yu Kuai <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[axboe: massage commit message]
Signed-off-by: Jens Axboe <[email protected]>

Author: Rajeev Mishra

drivers/block/loop.c

@@ -139,9 +139,20 @@ static int part_shift;
 
 static loff_t lo_calculate_size(struct loop_device *lo, struct file *file)
 {
+	struct kstat stat;
 	loff_t loopsize;
-	/* Compute loopsize in bytes */
-	loopsize = i_size_read(file->f_mapping->host);
+	int ret;
+
+	/*
+	 * Get the accurate file size. This provides better results than
+	 * cached inode data, particularly for network filesystems where
+	 * metadata may be stale.
+	 */
+	ret = vfs_getattr_nosec(&file->f_path, &stat, STATX_SIZE, 0);
+	if (ret)
+		return 0;
+
+	loopsize = stat.size;
 	if (lo->lo_offset > 0)
 		loopsize -= lo->lo_offset;
 	/* offset is beyond i_size, weird but possible */