btrfs blocks root from writing even when there is plenty of statfs->f_bfree

[Jeff Layton <jlayton@kernel.org>]

I've been doing some testing with btrfs exported via nfsd and hit this
bug. root is unable to write to the filesystem, even though statfs
reports that there should be 400k+ blocks available.

The reproducer is pretty simple. First I fill up the filesystem as an
unprivileged user (let this run until I hit ENOSPC):

[vagrant@kdevops-nfs-default btrfs]$ dd if=/dev/urandom of=/media/btrfs/bigfile bs=1M

...and then try to write to a file on the fs as root:

[root@kdevops-nfs-default btrfs]# strace -f -e statfs df .
statfs(".", {f_type=BTRFS_SUPER_MAGIC, f_bsize=4096, f_blocks=26214400, f_bfree=452204, f_bavail=0, f_files=0, f_ffree=0, f_fsid={val=[0xa9d43863, 0xb08b34cc]}, f_namelen=255, f_frsize=4096, f_flags=ST_VALID|ST_RELATIME}) = 0
Filesystem     1K-blocks      Used Available Use% Mounted on
/dev/nvme2n1   104857600 103048784         0 100% /media/btrfs
+++ exited with 0 +++
[root@kdevops-nfs-default btrfs]# xfs_io -f -c "pwrite -b 4096 4096 4096" ./file1
pwrite: No space left on device

This works on ext4 and xfs. The kernel is Linus' master branch as of a
couple of days ago:

    6.5.0-rc7-g081b0d4bef5d

Let me know if you need more info.
-- 
Jeff Layton <jlayton@kernel.org>