quoted

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.

quoted

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
http://marc.info/?l=linux-fsdevel&m=132585911925796&w=2

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?

Regards,
Surbhi.