[PATCH 5.7 018/204] btrfs: dont traverse into the seed devices in show_devname

Greg Kroah-Hartman
2020-08-20T09:35:35Z

From: Anand Jain <anand.jain@oracle.com>

commit 4faf55b03823e96c44dc4e364520000ed3b12fdb upstream.

->show_devname currently shows the lowest devid in the list. As the seed
devices have the lowest devid in the sprouted filesystem, the userland
tool such as findmnt end up seeing seed device instead of the device from
the read-writable sprouted filesystem. As shown below.

mount /dev/sda /btrfs
mount: /btrfs: WARNING: device write-protected, mounted read-only.

findmnt --output SOURCE,TARGET,UUID /btrfs
SOURCE TARGET UUID
/dev/sda /btrfs 899f7027-3e46-4626-93e7-7d4c9ad19111

btrfs dev add -f /dev/sdb /btrfs

umount /btrfs
mount /dev/sdb /btrfs

findmnt --output SOURCE,TARGET,UUID /btrfs
SOURCE TARGET UUID
/dev/sda /btrfs 899f7027-3e46-4626-93e7-7d4c9ad19111

All sprouts from a single seed will show the same seed device and the
same fsid. That's confusing.
This is causing problems in our prototype as there isn't any reference
to the sprout file-system(s) which is being used for actual read and
write.

This was added in the patch which implemented the show_devname in btrfs
commit 9c5085c14798 ("Btrfs: implement ->show_devname").
I tried to look for any particular reason that we need to show the seed
device, there isn't any.

So instead, do not traverse through the seed devices, just show the
lowest devid in the sprouted fsid.

After the patch:

mount /dev/sda /btrfs
mount: /btrfs: WARNING: device write-protected, mounted read-only.

findmnt --output SOURCE,TARGET,UUID /btrfs
SOURCE TARGET UUID
/dev/sda /btrfs 899f7027-3e46-4626-93e7-7d4c9ad19111

btrfs dev add -f /dev/sdb /btrfs
mount -o rw,remount /dev/sdb /btrfs

findmnt --output SOURCE,TARGET,UUID /btrfs
SOURCE TARGET UUID
/dev/sdb /btrfs 595ca0e6-b82e-46b5-b9e2-c72a6928be48

mount /dev/sda /btrfs1
mount: /btrfs1: WARNING: device write-protected, mounted read-only.

btrfs dev add -f /dev/sdc /btrfs1

findmnt --output SOURCE,TARGET,UUID /btrfs1
SOURCE TARGET UUID
/dev/sdc /btrfs1 ca1dbb7a-8446-4f95-853c-a20f3f82bdbb

cat /proc/self/mounts | grep btrfs
/dev/sdb /btrfs btrfs rw,relatime,noacl,space_cache,subvolid=5,subvol=/ 0 0
/dev/sdc /btrfs1 btrfs ro,relatime,noacl,space_cache,subvolid=5,subvol=/ 0 0

Reported-by: Martin K. Petersen <martin.petersen@oracle.com>
CC: stable@vger.kernel.org # 4.19+
Tested-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/btrfs/super.c | 21 +++++++--------------
1 file changed, 7 insertions(+), 14 deletions(-)

--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -2294,9 +2294,7 @@ static int btrfs_unfreeze(struct super_b
static int btrfs_show_devname(struct seq_file *m, struct dentry *root)
{
struct btrfs_fs_info *fs_info = btrfs_sb(root->d_sb);
- struct btrfs_fs_devices *cur_devices;
struct btrfs_device *dev, *first_dev = NULL;
- struct list_head *head;

/*
* Lightweight locking of the devices. We should not need
@@ -2306,18 +2304,13 @@ static int btrfs_show_devname(struct seq
* least until the rcu_read_unlock.
*/
rcu_read_lock();
- cur_devices = fs_info->fs_devices;
- while (cur_devices) {
- head = &cur_devices->devices;
- list_for_each_entry_rcu(dev, head, dev_list) {
- if (test_bit(BTRFS_DEV_STATE_MISSING, &dev->dev_state))
- continue;
- if (!dev->name)
- continue;
- if (!first_dev || dev->devid < first_dev->devid)
- first_dev = dev;
- }
- cur_devices = cur_devices->seed;
+ list_for_each_entry_rcu(dev, &fs_info->fs_devices->devices, dev_list) {
+ if (test_bit(BTRFS_DEV_STATE_MISSING, &dev->dev_state))
+ continue;
+ if (!dev->name)
+ continue;
+ if (!first_dev || dev->devid < first_dev->devid)
+ first_dev = dev;
}

if (first_dev)