On 28.10.21 г. 10:57, Anand Jain wrote:

On 27/10/2021 21:53, Nikolay Borisov wrote:

quoted

Currently balance is set as a running exclusive op in
btrfs_recover_balance in case of remount and a paused balance. That's a
bit eager because btrfs_recover_balance executes always and is not
affected by the 'skip_balance' mount option. This can lead to cases in
which a user has mounted the filesystem with 'skip_balance' due to
running out of space yet is unable to add a device to rectify the ENOSPC
because balance is set as a running exclusive op.

Fix this by setting balance in btrfs_resume_balance_async which takes
into consideration whether 'skip_balance' has been added or not.

Hmm, no. I roughly remember it was purposefully to avoid replacing
intervened with the half-completed/paused balance. As below.
Not sure if it is ok now?

Before patch: Can't replace with a paused balance.

------------
$ btrfs bal start --full-balance /btrfs
balance paused by user

$ btrfs rep start /dev/vg/scratch1 /dev/vg/scratch0 /btrfs
ERROR: unable to start device replace, another exclusive operation
'balance' in progress

$ mount -o remount,ro /dev/vg/scratch1 /btrfs
$ mount -o remount,rw,skip_balance /dev/vg/scratch1 /btrfs

$ btrfs rep start /dev/vg/scratch1 /dev/vg/scratch0 /btrfs
ERROR: unable to start device replace, another exclusive operation
'balance' in progress

$ umount /btrfs
$ mount -o skip_balance /dev/vg/scratch1 /btrfs

$ btrfs rep start /dev/vg/scratch1 /dev/vg/scratch0 /btrfs
ERROR: unable to start device replace, another exclusive operation
'balance' in progress
------------


After patch: Replace is successful even if there is a paused balance.

That's a fair point, so the patch needs to be augmented such that only
ADD is allowed but other exclusive ops are not. I will look into it.

<snip>