Re: [RFC] btrfs: Allow read-only mount with corrupted extent tree

["Dāvis Mosāns" <davispuh@gmail.com>]

ceturtd., 2021. g. 18. marts, plkst. 01:49 — lietotājs Qu Wenruo
(<quwenruo.btrfs@gmx.com>) rakstīja:
>
>
>
> On 2021/3/18 上午5:03, Dāvis Mosāns wrote:
> > trešd., 2021. g. 17. marts, plkst. 12:28 — lietotājs Qu Wenruo
> > (<quwenruo.btrfs@gmx.com>) rakstīja:
> >>
> >>
> >>
> >> On 2021/3/17 上午9:29, Dāvis Mosāns wrote:
> >>> trešd., 2021. g. 17. marts, plkst. 03:18 — lietotājs Dāvis Mosāns
> >>> (<davispuh@gmail.com>) rakstīja:
> >>>>
> >>>> Currently if there's any corruption at all in extent tree
> >>>> (eg. even single bit) then mounting will fail with:
> >>>> "failed to read block groups: -5" (-EIO)
> >>>> It happens because we immediately abort on first error when
> >>>> searching in extent tree for block groups.
> >>>>
> >>>> Now with this patch if `ignorebadroots` option is specified
> >>>> then we handle such case and continue by removing already
> >>>> created block groups and creating dummy block groups.
> >>>>
> >>>> Signed-off-by: Dāvis Mosāns <davispuh@gmail.com>
> >>>> ---
> >>>>    fs/btrfs/block-group.c | 14 ++++++++++++++
> >>>>    fs/btrfs/disk-io.c     |  4 ++--
> >>>>    fs/btrfs/disk-io.h     |  2 ++
> >>>>    3 files changed, 18 insertions(+), 2 deletions(-)
> >>>>
> >>>> diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
> >>>> index 48ebc106a606..827a977614b3 100644
> >>>> --- a/fs/btrfs/block-group.c
> >>>> +++ b/fs/btrfs/block-group.c
> >>>> @@ -2048,6 +2048,20 @@ int btrfs_read_block_groups(struct btrfs_fs_info *info)
> >>>>           ret = check_chunk_block_group_mappings(info);
> >>>>    error:
> >>>>           btrfs_free_path(path);
> >>>> +
> >>>> +       if (ret == -EIO && btrfs_test_opt(info, IGNOREBADROOTS)) {
> >>>> +               btrfs_put_block_group_cache(info);
> >>>> +               btrfs_stop_all_workers(info);
> >>>> +               btrfs_free_block_groups(info);
> >>>> +               ret = btrfs_init_workqueues(info, NULL);
> >>>> +               if (ret)
> >>>> +                       return ret;
> >>>> +               ret = btrfs_init_space_info(info);
> >>>> +               if (ret)
> >>>> +                       return ret;
> >>>> +               return fill_dummy_bgs(info);
> >>
> >> When we hit bad things in extent tree, we should ensure we're mounting
> >> the fs RO, or we can't continue.
> >>
> >> And we should also refuse to mount back to RW if we hit such case, so
> >> that we don't need anything complex, just ignore the whole extent tree
> >> and create the dummy block groups.
> >>
> >
> > That's what we're doing here, `ignorebadroots` implies RO mount and
> > without specifying it doesn't mount at all.
> >
> >>>
> >>> This isn't that nice, but I don't really know how to properly clean up
> >>> everything related to already created block groups so this was easiest
> >>> way. It seems to work fine.
> >>> But looks like need to do something about replay log aswell because if
> >>> it's not disabled then it fails with:
> >>>
> >>> [ 1397.246869] BTRFS info (device sde): start tree-log replay
> >>> [ 1398.218685] BTRFS warning (device sde): sde checksum verify failed
> >>> on 21057127661568 wanted 0xd1506ed9 found 0x22ab750a level 0
> >>> [ 1398.218803] BTRFS warning (device sde): sde checksum verify failed
> >>> on 21057127661568 wanted 0xd1506ed9 found 0x7dd54bb9 level 0
> >>> [ 1398.218813] BTRFS: error (device sde) in __btrfs_free_extent:3054:
> >>> errno=-5 IO failure
> >>> [ 1398.218828] BTRFS: error (device sde) in
> >>> btrfs_run_delayed_refs:2124: errno=-5 IO failure
> >>> [ 1398.219002] BTRFS: error (device sde) in btrfs_replay_log:2254:
> >>> errno=-5 IO failure (Failed to recover log tree)
> >>> [ 1398.229048] BTRFS error (device sde): open_ctree failed
> >>
> >> This is because we shouldn't allow to do anything write to the fs if we
> >> have anything wrong in extent tree.
> >>
> >
> > This is happening when mounting read-only. My assumption is that it
> > only tries to replay in memory without writing anything to disk.
> >
>
> We lacks the check on log tree.
>
> Normally for such forced RO mount, log replay is not allowed.
>
> We should output a warning to prompt user to use nologreplay, and reject
> the mount.
>

I'm not familiar with log replay but couldn't there be something
useful (ignoring ref counts) that would still be worth replaying in
memory?