BZ#101951, Overlayfs on top of btrfs causes kernel oops + freeze

[Colin Ian King <colin.king@canonical.com>]

Hi there,

bug: https://bugzilla.kernel.org/show_bug.cgi?id=101951 and also
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1532145

Commit 4bacc9c9234c7c8eec44f5ed4e960d9f96fa0f01 ("overlayfs: Make f_path
always point to the overlay and f_inode to the underlay") resulted in an
issue when using a combination of btrfs and overlayfs.  This is
noticeable when doing a fsync() on a file in a chroot with overlayfs on
top of btrfs; we hit a kernel oops in btrfs_sync_file() on
atomic_inc(&root->log_batch) because root is NULL.

I've debugged this further and found that in btrfs_sync_file():

	struct inode *inode = d_inode(dentry);

does not return the inode I expected when using the stacked overlay fs,
where as:

	struct inode *inode = file_inode(file);

does.

However, I'm not well at all well versed in btrfs, so I am not confident
this is a actually correct.  Any comments?

Colin