Re: [PATCH 0/3] btrfs: allow BTRFS_IOC_SNAP_DESTROY_V2 to remove ghost subvolume

On Tue, Jul 20, 2021 at 12:33:47PM +0800, Qu Wenruo wrote:

On 2021/7/20 下午12:05, Zygo Blaxell wrote:

quoted

On Mon, Jun 28, 2021 at 06:16:34PM +0800, Qu Wenruo wrote:

quoted

Since we're busting ghost subvolumes, the branch is now called
ghost_busters:
https://github.com/adam900710/linux/tree/ghost_busters

The first two patches are just cleanup found during the development.

The first is a missing check for subvolid range, the missing check
itself won't cause any harm, just returning -ENOENT from dentry lookup,
other than the expected -EINVAL.

The 2nd is a super old dead comment from the early age of btrfs.

The final patch is the real work to allow patched "btrfs subvolume delete -i"
to delete ghost subvolume.
Tested with the image dump of previous submitted btrfs-progs patchset.

Qu Wenruo (3):
   btrfs: return -EINVAL if some user wants to remove uuid/data_reloc
     tree
   btrfs: remove dead comment on btrfs_add_dead_root()
   btrfs: allow BTRFS_IOC_SNAP_DESTROY_V2 to remove ghost subvolume

I hit this bug on several machines while they were running 5.11.  The
ghost subvols seem to occur naturally--I didn't change my usual workloads
to get them, they just showed up in fairly normal snapshot rotation.

And there is no powerloss involved?

There could be.  The test VMs have frequent simulated powerloss events
because we're testing for regressions in post-powerloss behavior (I
guess this is one?).  It also affected laptops that probably did have
a forced shutdown or two (WiFi and Bluetooth are huge crash generators
on new hardware).

Nothing running 5.11 on stable power seems to have been affected so far.

Then it's a much serious problem than I thought.

Thanks,
Qu

quoted

They don't seem to occur on 5.10 (up to .46) or on 5.12 and later, but
once they are created, they don't go away without using this patch to
remove them.

It's odd that it doesn't seem to happen on kernels other than 5.11.
That would suggest it was broken in 5.11-rc and fixed in 5.12, which
might be a useful range to search for regressions and accidental fixes.

In any case, if a user has been running 5.11, they're going to have
ghost subvols lying around on their filesystem, so we might as well get
ready to deal with them in the kernel and tools.

quoted

This patch does get rid of the ghost subvols after the fact, quite nicely.

Some users on IRC have hit the same problem.  One was running Debian's
backported 5.10, which doesn't fit the pattern of kernel versions I've
observed, but maybe Debian backported something?

quoted

  fs/btrfs/ioctl.c       | 81 +++++++++++++++++++++++++++++++++++++++++-
  fs/btrfs/transaction.c |  7 ++--
  2 files changed, 84 insertions(+), 4 deletions(-)

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2.32.0