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

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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.

Thanks,
Qu