Re: [PATCH v2 1/7] Adding support to freeze and unfreeze a journal

[Surbhi Palande]

Hi Jan,

>>
>>
>> If all the write operations were journaled, then this patch would not allow
>> ext4 filesystem to have any dirty data after its frozen.
>> (as journal_start() would block).
>>
>> ÂI think the only one candidate that creates dirty data without calling
>> ext4_journal_start() is mmapped?
> ÂNo, the problem is in any write path. The problem is with operations
> that happen during the phase when s_frozen == SB_FREEZE_WRITE. These
> operations dirty the filesystem but running sync may easily miss them.
> During this phase journal is not frozen so that does not help you in any
> way.
>
> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â ÂHonza

Ok! No new transaction can really start after the journal is frozen.
But we can have dirty data after SB_FREEZE_WRITE and before
SB_FREEZE_TRANS.
I agree with you. However, can this be fixed by adding a
sync_filesystem() in freeze_super() after the sb->s_op->freeze_fs() is
over?


So then essentially, when freeze_super() returns, the page cache is clean?

I do definitely agree that the fix is to add a lock for mutual
exclusion between freeze filesystem and writes to a frozen filesystem.

Thanks!

Regards,
Surbhi.
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