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

[Boris Burkov <boris@bur.io>]

On Sun, Sep 06, 2026 at 04:33:36PM +0930, Qu Wenruo wrote:
> [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.
> 

I generally really like these patches! I sent some ideas for what I
think would be improvements to each, but if you don't like my ideas and
just want to land this, please feel free to add

Reviewed-by: Boris Burkov <boris@bur.io>

Thanks,
Boris

> 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
>