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

[Surbhi Palande]

> > Hrm let me think through this a little more; we actually do:
> >
> > t16) ext4_journal_start()
> > Â t17) ext4_journal_start_sb()
> > Â Â t18) handle = ext4_journal_current_handle();
> > Â Â t19) if (!handle) vfs_check_frozen()
> > Â Â t20) ... jbd2_journal_start()
> ÂAh, 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? ÂSo 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...?
> ÂWell, the problem with running operations and freezing is more
> fundamental I believe. See my email
> http://marc.info/?l=linux-fsdevel&m=132585911925796&w=2
>
> So I believe we'll need some better exclusion mechanism already in VFS.
>
> 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
>

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?

Regards,
Surbhi.
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