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Subject: Re: [RFC] btrfs: Allow read-only mount with corrupted extent tree
To:     =?UTF-8?B?RMSBdmlzIE1vc8SBbnM=?= <davispuh@gmail.com>
Cc:     Btrfs BTRFS <linux-btrfs@vger.kernel.org>, clm@fb.com,
        Josef Bacik <josef@toxicpanda.com>, dsterba@suse.com,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Zygo Blaxell <ce3g8jdj@umail.furryterror.org>
References: <20210317012054.238334-1-davispuh@gmail.com>
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 <20a5d997-740a-ca57-8cbc-b88c1e34c8fc@gmx.com>
 <CAOE4rSyX-qTWKS_MTS5dLpfuVnqS=LwfqThyCTP=iBEH5x2bOQ@mail.gmail.com>
 <01129192-1b93-2a93-2edd-f29f544fe340@gmx.com>
 <CAOE4rSwOLQY1JWr-Mdq06Y9nwU_WcQBnfXZx3VWhRQGnBThHUQ@mail.gmail.com>
From:   Qu Wenruo <quwenruo.btrfs@gmx.com>
Message-ID: <f48c758a-39a3-c73e-fc50-5ab37d2280f9@gmx.com>
Date:   Sat, 20 Mar 2021 08:34:14 +0800
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On 2021/3/19 =E4=B8=8B=E5=8D=8811:34, D=C4=81vis Mos=C4=81ns wrote:
> ceturtd., 2021. g. 18. marts, plkst. 01:49 =E2=80=94 lietot=C4=81js Qu W=
enruo
> (<quwenruo.btrfs@gmx.com>) rakst=C4=ABja:
>>
>>
>>
>> On 2021/3/18 =E4=B8=8A=E5=8D=885:03, D=C4=81vis Mos=C4=81ns wrote:
>>> tre=C5=A1d., 2021. g. 17. marts, plkst. 12:28 =E2=80=94 lietot=C4=81js=
 Qu Wenruo
>>> (<quwenruo.btrfs@gmx.com>) rakst=C4=ABja:
>>>>
>>>>
>>>>
>>>> On 2021/3/17 =E4=B8=8A=E5=8D=889:29, D=C4=81vis Mos=C4=81ns wrote:
>>>>> tre=C5=A1d., 2021. g. 17. marts, plkst. 03:18 =E2=80=94 lietot=C4=81=
js D=C4=81vis Mos=C4=81ns
>>>>> (<davispuh@gmail.com>) rakst=C4=ABja:
>>>>>>
>>>>>> 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: D=C4=81vis Mos=C4=81ns <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 =3D check_chunk_block_group_mappings(info);
>>>>>>     error:
>>>>>>            btrfs_free_path(path);
>>>>>> +
>>>>>> +       if (ret =3D=3D -EIO && btrfs_test_opt(info, IGNOREBADROOTS)=
) {
>>>>>> +               btrfs_put_block_group_cache(info);
>>>>>> +               btrfs_stop_all_workers(info);
>>>>>> +               btrfs_free_block_groups(info);
>>>>>> +               ret =3D btrfs_init_workqueues(info, NULL);
>>>>>> +               if (ret)
>>>>>> +                       return ret;
>>>>>> +               ret =3D 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 mountin=
g
>>>> 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 tre=
e
>>>> 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 easie=
st
>>>>> 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 faile=
d
>>>>> on 21057127661568 wanted 0xd1506ed9 found 0x22ab750a level 0
>>>>> [ 1398.218803] BTRFS warning (device sde): sde checksum verify faile=
d
>>>>> on 21057127661568 wanted 0xd1506ed9 found 0x7dd54bb9 level 0
>>>>> [ 1398.218813] BTRFS: error (device sde) in __btrfs_free_extent:3054=
:
>>>>> errno=3D-5 IO failure
>>>>> [ 1398.218828] BTRFS: error (device sde) in
>>>>> btrfs_run_delayed_refs:2124: errno=3D-5 IO failure
>>>>> [ 1398.219002] BTRFS: error (device sde) in btrfs_replay_log:2254:
>>>>> errno=3D-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 rejec=
t
>> 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?
>
Log replay means metadata write.

Any write needs a valid extent tree to find out free space for new
metadata/data.

So no, we can't do anything but completely ignoring the log.

Thanks,
Qu