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Subject[PATCH v2 0/6] Btrfs: show subvolume name and ID in /proc/mounts
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Here's version 2 of providing the subvolume name and ID in /proc/mounts.

It turns out that getting the name of a subvolume reliably is a bit
trickier than it would seem because of how mounting subvolumes by ID is
implemented. In particular, in that case, the dentry we get for the root
of the mount is not necessarily attached to the dentry tree, which means
that the obvious solution of just dumping the dentry does not work. The
solution I put together makes the tradeoff of churning a bit more code
in order to avoid implementing this with weird hacks.

Changes from v1 (https://lkml.org/lkml/2015/4/8/16):

- Put subvol= last in show_options
- Change commit log to remove comment about userspace having no way to
know which subvolume is mounted, as David pointed out you can use
btrfs inspect-internal rootid <mountpoint>
- Split up patch 2
- Minor coding style fixes

This still applies to v4.0-rc7. Tested manually and with the script
below (updated from v1).

Thanks!

Omar Sandoval (6):
Btrfs: lock superblock before remounting for rw subvol
Btrfs: remove all subvol options before mounting top-level
Btrfs: clean up error handling in mount_subvol()
Btrfs: fail on mismatched subvol and subvolid mount options
Btrfs: unify subvol= and subvolid= mounting
Btrfs: show subvol= and subvolid= in /proc/mounts

fs/btrfs/super.c | 376 ++++++++++++++++++++++++++++++++++++-------------------
fs/seq_file.c | 1 +
2 files changed, 251 insertions(+), 126 deletions(-)

Testing script:

----
#!/bin/sh

set -e

check_subvol () {
NAME="$1"
ID="$2"

# Mount by name.
mount -osubvol="$NAME" /dev/vdb /mnt
if ! findmnt -fno OPTIONS /mnt | grep -F ",subvolid=$ID,subvol=$NAME"; then
echo "Failed $NAME" >&2
umount /mnt
exit 1
fi
umount /mnt

# Mount by ID.
mount -osubvolid="$ID" /dev/vdb /mnt
if ! findmnt -fno OPTIONS /mnt | grep -F ",subvolid=$ID,subvol=$NAME"; then
echo "Failed $ID" >&2
umount /mnt
exit 1
fi
umount /mnt
}

check_default_subvol () {
NAME="$1"
ID="$2"

mount /dev/vdb /mnt
if ! findmnt -fno OPTIONS /mnt | grep -F ",subvolid=$ID,subvol=$NAME"; then
echo "Failed default" >&2
umount /mnt
exit 1
fi
umount /mnt
}

mkfs.btrfs -f /dev/vdb
mount /dev/vdb /mnt
btrfs subvolume create /mnt/vol
btrfs subvolume create /mnt/vol/nestedvol
mkdir /mnt/dir
btrfs subvolume create /mnt/dir/dirvol
btrfs subvolume create /mnt/dir/dirvol/nesteddirvol
mkdir /mnt/vol/voldir
btrfs subvolume create /mnt/vol/voldir/voldirvol
btrfs subvolume list /mnt
umount /mnt

check_subvol /vol 257
check_subvol /vol/nestedvol 258
check_subvol /dir/dirvol 259
check_subvol /dir/dirvol/nesteddirvol 260
check_subvol /vol/voldir/voldirvol 261

check_default_subvol / 5

mount /dev/vdb /mnt
btrfs subvolume set-default 257 /mnt
umount /mnt

check_default_subvol /vol 257

if mount -osubvolid=258,subvol=/vol /dev/vdb /mnt 2>/dev/null; then
umount /mnt
echo "Mount of mismatched subvol and subvolid should have failed" >&2
exit 1
fi
----

--
2.3.5



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