linux-kernel - Re: 2.6.18 ext3 panic.

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Date:	Thu, 12 Oct 2006 14:28:20 +0200
From:	Jan Kara <jack@...e.cz>
To:	Eric Sandeen <esandeen@...hat.com>
Cc:	Badari Pulavarty <pbadari@...ibm.com>,
	Eric Sandeen <sandeen@...deen.net>,
	Dave Jones <davej@...hat.com>, Andrew Morton <akpm@...l.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: 2.6.18 ext3 panic.

> Badari Pulavarty wrote:
> 
> >Here is what I think is happening..
> >
> >journal_unmap_buffer() - cleaned the buffer, since its outside EOF, but
> >its a part of the same page. So it remained on the page->buffers
> >list. (at this time its not part of any transaction).
> >
> >Then, ordererd_commit_write() called journal_dirty_data() and we added
> >all these buffers to BJ_SyncData list. (at this time buffer is clean -
> >not dirty).
> >
> >Now msync() called __set_page_dirty_buffers() and dirtied *all* the
> >buffers attached to this page.
> >
> >journal_submit_data_buffers() got around to this buffer and tried to
> >submit the buffer...
  Yes, this is certainly one we need to fix.

> This seems about right, but one thing bothers me in the traces; it seems 
> like there is some locking that is missing.  In
> http://people.redhat.com/esandeen/traces/eric_ext3_oops1.txt
> for example, it looks like journal_dirty_data gets started, but then the 
> buffer_head is acted on by journal_unmap_buffer, which decides this buffer 
> is part of the running transaction, past EOF, and clears mapped, dirty, 
  It's part of the committing transaction.

> etc.  Then journal_dirty_data picks up again, decides that the buffer is 
> not on the right list (now BJ_None) and puts it back on BJ_SyncData.  Then 
> it gets picked up by journal_submit_data_buffers and submitted, and oops.
  Now it is put on the running transaction's BJ_SyncData list. But
otherwise you're right.

> Talking with Stephen, it seemed like the page lock should synchronize these 
> threads, but I've found that we can get to journal_dirty_data acting on the 
> buffer heads w/o having the page locked...
  Yes, PageLock should protect us. Where can we call
journal_dirty_data() without PageLock? I see the following callers:
  ext3_ordered_commit_write - should have PageLock
  ext3_ordered_writepage - has PageLock
  ext3_block_truncate_page - has PageLock

  And that are all callers from ext3. Am I missing something?

								Honza
-- 
Jan Kara <jack@...e.cz>
SuSE CR Labs
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