Re: [PATCH] xfs: di_flushiter considered harmful

Mark Tinguely
2013-07-22T16:38:01Z

On 07/22/13 10:15, Markus Trippelsdorf wrote:
> On 2013.07.22 at 09:40 -0500, Mark Tinguely wrote:
>> On 07/22/13 06:07, Markus Trippelsdorf wrote:
>>> On 2013.07.22 at 20:18 +1000, Dave Chinner wrote:
>>>> From: Dave Chinner<dchinner@redhat.com>
>>>>
>>>> When we made all inode updates transactional, we no longer needed
>>>> the log recovery detection for inodes being newer on disk than the
>>>> transaction being replayed - it was redundant as replay of the log
>>>> would always result in the latest version of the inode woul dbe on
>>>> disk. It was redundant, but left in place because it wasn't
>>>> considered to be a problem.
>>>>
>>>> However, with the new "don't read inodes on create" optimisation,
>>>> flushiter has come back to bite us. Essentially, the optimisation
>>>> made always initialises flushiter to zero in the create transaction,
>>>> and so if we then crash and run recovery and the inode already on
>>>> disk has a non-zero flushiter it will skip recovery of that inode.
>>>> As a result, log recovery does the wrong thing and we end up with a
>>>> corrupt filesystem.
>>>>
>>>> Because we have to support old kernel to new kernl upgrades, we
>>>> can't just get rid of the flushiter support in log recovery as we
>>>> might be upgrading from a kernel that doesn't have fully transaction
>>>> inode updates. Unfortunately, for v4 superblocks there is no way to
>>>> guarantee that log recovery knows about this fact.
>>>>
>>>> We cannot add a new inode format flag to say it's a "special inode
>>>> create" because it won't be understood by older kernels and so
>>>> recovery could do the wrong thing on downgrade. We cannot specially
>>>> detect the combination of zero mode/non-zero flushiter on disk to
>>>> non-zero mode, zero flushiter in the log item during recovery
>>>> because wrapping of the flushiter can result in false detection.
>>>>
>>>> Hence that makes this "don't use flushiter" optimisation limited to
>>>> a disk format that guarantees that we don't need it. And that means
>>>> the only fix here is to limit the "no read IO on create"
>>>> optimisation to version 5 superblocks....
>>>
>>> I think your patch misses the following part:
>>>
>>
>>
>> Dave's patch is limited to the new v5 (crc) superblock. The constraints
>> that has to be dealt with are in the commit message as to why it is
>> limited to the new v5 superblock.
>>
>> Going back to your 07/10/2013 message, your filesystem is:
>>
>> /dev/root on / type xfs (rw,relatime,attr2,inode64,logbsize=256k,noquota)
>>
>> or the non-crc v4 superblock with inode 2 that is probably why it is
>> still failing for you.
>>
>> It seems to me that since we cannot fix this for inode 1/2, then besides
>> this patch we have to revert patch cca9f93a52d and make it inode 3+ /
>> superblock 5+ (crc) dependent.
>
> Which is exactly what the hunk I've posted does.
>
> Here's the combined patch:

Thank-you for the clarification.

--Mark.