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

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