Re: [PATCH 0/4] btrfs: rescue= mount options enhancement to support interrupted csum conversion

[Josef Bacik <josef@toxicpanda.com>]

On Tue, Jun 11, 2024 at 02:51:34PM +0930, Qu Wenruo wrote:
> [REPO]
> https://github.com/adam900710/linux/tree/rescue_changes
> 
> [BACKGROUND]
> There is an adventurous user using btrfstune to convert a 32T btrfs
> from crc32c to xxhash.
> 
> However for such huge fs, it takes too long time and the reporter
> canceled the conversion.
> 
> This makes the reporter unable to mount the fs at all.
> 
> [CAUSE]
> First of all, for a half converted fs, we will never allow RW mount, so
> everything must be done in rescue mode.
> 
> There are several different stages of csum conversion, and at different
> stage it requires different handling from kernel:
> 
> - Generationg new data csums
>   At this stage only the super flags (CHANGING_DATA_CSUM flag) is
>   preventing the kernel from mounting.
> 
>   Intrdoce "rescue=ignoresuperflags" to address this.
> 
> - Deleting old data cums
>   The same super flags problem, with possible missing data csums.
> 
>   Despite the new "rescue=ignoresuperflags", end users will also need
>   the existing "rescue=ignoredatacsums" mount option.
> 
> - Renaming the objectid of new data cums
>   The new csums' objectid will be changed to the regular one.
> 
>   During this we can hit data csum mismatch.
>   So the same "rescue=ignoresuperflags:ignoredatacsums" can handle it
>   already.
> 
> - Rewriting metadata csums
>   This part is done in-place (no COW), with a new super flags
>   (CHANGING_META_CSUM).
> 
>   So here introduce a new "rescue=ignoremetacsums" to ignore the
>   metadata checksum verification (and rely on the remaining sanity
>   checks like tree-checkers).
> 
> The first 2 patches are just small cleanups, meanwhile the last two are
> the new "rescue=" mount options to handle interrupted csum change.
> 
> Qu Wenruo (4):
>   btrfs: remove unused Opt enums
>   btrfs: output the unrecognized super flags as hex
>   btrfs: introduce new "rescue=ignoremetacsums" mount option
>   btrfs: introduce new "rescue=ignoresuperflags" mount option
> 

Reviewed-by: Josef Bacik <josef@toxicpanda.com>

Could you update the docs in btrfs-progs/wherever we're keeping the readthedocs
stuff so that these new flags are documented?  Thanks,

Josef