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Date:	Sun, 14 Aug 2011 22:06:52 -0400
From:	Ted Ts'o <tytso@....edu>
To:	Curt Wohlgemuth <curtw@...gle.com>
Cc:	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] jbd2: instrument jh on wrong transaction BUG_ON's

On Sun, Aug 14, 2011 at 01:01:01PM -0700, Curt Wohlgemuth wrote:
> 
> It seems that  jh->b_transaction is NULL, and deferencing it to print
> out jh->b_transaction->t_tid causes the NULL pointer deref.

Hmm, if jh->b_transaction is null, then
jbd2_journal_get_write_access() must not have been called on the
journal_head.  But the relevant code from ext4_journalled_writepage() is:

	ret = walk_page_buffers(handle, page_bufs, 0, len, NULL,
				do_journal_get_write_access);

	err = walk_page_buffers(handle, page_bufs, 0, len, NULL,
				write_end_fn);

(write_end_fn is the function which calls which ultimately calls
ext4_handle_dirty_metadata which then calls
jbd2_journal_dirty_metadata(), which is where you're seeing the BUG_ON).

I suspect what is going on is do_get_write_access() is returning an
error, which means we never set jh->b_transaction.  Hence when we call
jbd2_journal_data_metadata(), we trigger the BUG.

OK, what could cause do_get_write_access() to return an error?  Two
conditions: if the handle is aborted (due to a previous error), in
which case it returns -EROFS, or if it can't get the memory needed
make a copy of the buffer, in which case there should have been a
"do_get_write_access: OOM for frozen buffer" error message earlier in
the log.  (No, it's not prefixed with JBD2 --- it probably should be.)

Any chance you're seeing any indication of either possibility in the
messages before the BUG message?

						- Ted
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