[PATCH V3] btrfs: print error message if bdev_file_open_by_path function fails during mount.

[Li Zhang <zhanglikernel@gmail.com>]

[ENHANCEMENT]
When mounting a btrfs filesystem, the filesystem opens the
block device, and if this fails, there is no message about
it. print a message about it to help debugging.

[TEST]
I have a btrfs filesystem on three block devices,
one of which is write-protected, so regular mounts fail,
but there is no message in dmesg.

/dev/vdb normal
/dev/vdc write protected
/dev/vdd normal

Before patch:
$ sudo mount /dev/vdb /mnt/
mount: mount(2) failed: no such file or directory
$ sudo dmesg # Show only messages about missing block devices
....
[ 352.947196] BTRFS error (device vdb): devid 2 uuid 4ee2c625-a3b2-4fe0-b411-756b23e08533 missing
....

After patch:
$ sudo mount /dev/vdb /mnt/
mount: mount(2) failed: no such file or directory
$ sudo dmesg # Show bdev_file_open_by_path failed.
....
[ 352.944328] BTRFS error: failed to open device for path /dev/vdc with flags 0x3: -13
[ 352.947196] BTRFS error (device vdb): missing devid 2 uuid 4ee2c625-a3b2-4fe0-b411-756b23e08533
....

Signed-off-by: Li Zhang <zhanglikernel@gmail.com>
---

V1:
  Use printk to print messages

V2:
  Use btrfs_err to print messages and format output


V3:
  Fix typo
  Format description


 fs/btrfs/volumes.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index c39145e..315d35a 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -476,6 +476,7 @@ static noinline struct btrfs_fs_devices *find_fsid(
 
 	if (IS_ERR(*bdev_file)) {
 		ret = PTR_ERR(*bdev_file);
+		btrfs_err(NULL, "failed to open device for path %s with flags 0x%x: %d", device_path, flags, ret);
 		goto error;
 	}
 	bdev = file_bdev(*bdev_file);
-- 
1.8.3.1