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

["Dāvis Mosāns" <davispuh@gmail.com>]

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.