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SubjectRe: [PATCH v2 1/7] Adding support to freeze and unfreeze a journal
> > Hrm let me think through this a little more; we actually do:
> >
> > t16) ext4_journal_start()
> >   t17) ext4_journal_start_sb()
> >     t18) handle = ext4_journal_current_handle();
> >     t19) if (!handle) vfs_check_frozen()
> >     t20) ... jbd2_journal_start()
>  Ah, right. I forgot.
> > So actually we *do* block new handles, but let *existing* ones
> > continue (see commits 6b0310fbf087ad6e9e3b8392adca97cd77184084
> > and be4f27d324e8ddd57cc0d4d604fe85ee0425cba9)
> >
> > So your assertion that a new handle is started is incorrect
> > in general, isn't it?  So then does the fix seem necessary?
> > Or, at least, in the fashion below - maybe we need to just make
> > sure all started handles complete before the unlock_updates?
> > Or am I missing something...?
>  Well, the problem with running operations and freezing is more
> fundamental I believe. See my email
> So I believe we'll need some better exclusion mechanism already in VFS.
>                                                                Honza

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).

I think the only one candidate that creates dirty data without
calling ext4_journal_start() is mmapped?

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