[PATCH v2 0/2] btrfs: allow more fine control to rescue=usebackuproot

[Qu Wenruo <wqu@suse.com>]

[CHANGELOG]
v2:
- Fix a missing use_backup_slot update for rescue=all mount option
  Which can trigger the ASSERT() on use_backup_slot.

- Add proper output for btrfs_show_options()

- Slightly update the commit message of the 2nd patch
  To address a false alert from Sashiko, where it thinks it's a bug not
  to load any backup root for the newest slot.
  For the newest slot, it matches the current generation in the super
  block, thus every root should be the same as the super block, and no
  need to load the bytenr from backup.

There is a bug report that for a specific corrupted btrfs, the
"rescue=usebackuproot" still chose the newest slot (aka, the same tree
root as the one in the super block) to mount the fs, and resulted
transid mismatch.

Meanwhile the reporter used btrfs-mod-sb to modify the fs to use a
specific backup slot, then the fs can pass btrfs-check.

This shows the limit of the current automatic backup root detection,
that as long as all tree root nodes can be loaded, btrfs will consider
it as a valid backup slot, without trying any other slot.

And end user has no way to tell btrfs to use a specific slot.

This patchest address the problem by:

- Make "rescue=usebackuproot" to always use the second newest slot
  Which has the highest chance to still get every tree block right
  without transid error.

- Introduce new "rescue=usebackuproot_*" mount option
  Where "*" can be 0/1/2/3.
  0 means the newest slot (aka, the one matching the super block
  generation), 1/2/3 means the second/third/fourth(oldest) newest slot.

  Now "rescue=usebackuproot" is just the same as
  "rescue=usebackuproot_1".

Although those "rescue=usebackuproot*" mount options still requires full
RO.
For proper recovery, we still need to use "btrfs check", and a new
option for btrfs-check will be introduced soon to make the backuproot
usage simpler for progs.

Qu Wenruo (2):
  btrfs: always use the second newest slot for rescue=usebackuproot
  btrfs: introduce more accurate usebackuproot options

 fs/btrfs/disk-io.c | 106 ++++++++++++++++++++-------------------------
 fs/btrfs/fs.h      |   1 +
 fs/btrfs/super.c   |  28 +++++++++++-
 3 files changed, 74 insertions(+), 61 deletions(-)

-- 
2.55.0