Re: [PATCH 1/3] btrfs: Add incompat flags check for btrfs_check_super_valid()

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

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On 2018年04月21日 10:38, Anand Jain wrote:
> 
> 
> On 04/20/2018 11:15 PM, David Sterba wrote:
>> On Fri, Apr 20, 2018 at 10:32:03PM +0800, Anand Jain wrote:
>>>
>>>
>>> On 04/19/2018 05:38 PM, Qu Wenruo wrote:
>>>> Although we have already checked incompat flags manually before really
>>>> mounting it, we could still enhance btrfs_check_super_valid() to check
>>>> incompat flags for later write time super block validation check.
>>>>
>>>> This patch adds such incompat flags check for
>>>> btrfs_check_super_valid(),
>>>> currently it won't be triggered, but provides the basis for later write
>>>> time check.
>>>>
>>>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>>>> ---
>>>>    fs/btrfs/disk-io.c | 13 +++++++++++++
>>>>    1 file changed, 13 insertions(+)
>>>>
>>>> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
>>>> index 60caa68c3618..ec123158f051 100644
>>>> --- a/fs/btrfs/disk-io.c
>>>> +++ b/fs/btrfs/disk-io.c
>>>> @@ -4104,6 +4104,19 @@ static int btrfs_check_super_valid(struct
>>>> btrfs_fs_info *fs_info)
>>>>            ret = -EINVAL;
>>>>        }
>>>>    +    /*
>>>> +     * Before calling btrfs_check_super_valid() we have already
>>>> checked
>>>> +     * incompat flags. So if we developr new incompat flags, it's
>>>> must be
>>>> +     * some corruption.
>>>> +     */
>>>> +    if (btrfs_super_incompat_flags(sb) &
>>>> ~BTRFS_FEATURE_INCOMPAT_SUPP) {
>>>> +        btrfs_err(fs_info,
>>>> +        "corrupted incompat flags detected 0x%llx, supported 0x%llx",
>>
>>> 2707         features = btrfs_super_incompat_flags(disk_super) &
>>> 2708                 ~BTRFS_FEATURE_INCOMPAT_SUPP;
>>> 2709         if (features) {
>>> 2710                 btrfs_err(fs_info,
>>> 2711                     "cannot mount because of unsupported
>>> optional features (%llx)",
>>> 2712                     features);
>>> 2713                 err = -EINVAL;
>>> 2714                 goto fail_alloc;
>>> 2715         }
>>
>> So there's a "user experience" change, now that you pointed out the
>> other check. 
> 
>  Its a regression.
> 
>> If a filesystem with incompat bits set will be mounted,
>> this will say 'you have corrupted filesystem', which is not IMO what we
>> want to tell.
>>
>> Tough the extended btrfs_check_super_valid could catch the corrupted
>> incompat bits, what we need at mount time is wording from the 2nd
>> message ("cannot mount unsuppported features").
>>
>> I think that there should be a separate function that does the
>> pre-commit checks, calls btrfs_check_super_valid and also validates the
>> incompat bit.
> 
>  We can still print it as 'unsupported optional features', which would
>  imply corruption in the non-mount context.

In that case such wording is not obvious enough to info it's super block
corruption.

So I still prefer current way of checking incompact flags and csum types
manually at mount time before btrfs_validate_super().

David's idea of a separate function to do mount time check and gives
better prompt is pretty good.
Especially for incompat features, as incompat features will increase in
the future and for older kernel info such feature as corruption is not
acceptable.

But currently, there are only 2 members we need to check at mount time
(csum_type and incompat features), and only one caller, current code
looks good enough for its purpose.

Thanks,
Qu

> 
> Thanks, Anand
> 
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