linux-kernel - Re: [RFC][PATCH] set_page_buffer_dirty should skip unmapped buffers

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Message-ID: <20060906170330.GB14345@atrey.karlin.mff.cuni.cz>
Date:	Wed, 6 Sep 2006 19:03:30 +0200
From:	Jan Kara <jack@...e.cz>
To:	Badari Pulavarty <pbadari@...ibm.com>
Cc:	Andrew Morton <akpm@...l.org>,
	Anton Altaparmakov <aia21@....ac.uk>, sct@...hat.com,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [RFC][PATCH] set_page_buffer_dirty should skip unmapped buffers

> On Wed, 2006-09-06 at 18:27 +0200, Jan Kara wrote:
> > > 
> > > But my debug clearly shows that we are clearing the buffer, while
> > > we haven't actually submitted to ll_rw_block() code. (I added "track"
> > > flag to bh and set it in journal_commit_transaction() when we add
> > > them to wbuf[] and clear it in ll_rw_block() after submit. I checked
> > > for this flag in journal_unmap_buffer() while clearing the buffer).
> > > Here is what my debug shows:
> > > 
> > > buffer is tracked bh ffff8101686ea850 size 1024 
> > > 
> > > Call Trace:
> > >  [<ffffffff8020b395>] show_trace+0xb5/0x370
> > >  [<ffffffff8020b665>] dump_stack+0x15/0x20
> > >  [<ffffffff8030d474>] journal_invalidatepage+0x314/0x3b0
> >   I see just journal_invalidatepage() here. That is fine. It calls
> > journal_unmap_buffer() which should do nothing return 0. If it does
> > not it would be IMO bug.. If the buffer is really unmapped here, in what
> > state it is (i.e. which list is it on?).
> > 
> Acutally, I added dump_stack() in journal_unmap_buffer() when it
> does clear_buffer_mapped(). gcc must of pulled in the function ..
> I will add more debug to track the list bh came from.
  Ah, ok. My guess is that the buffer is in BJ_Locked list. But we don't
write buffers from BJ_Locked list (as they are supposedly already
written). What probably happens is:
  Process 1				Process 2
  We start scanning t_sync_datalist.
  Add buffer to wbuf[].
					Locks the buffer (it can be some other
					writer).
  Restarts scanning the list
  Finds buffer is locked -> moves to
   BJ_Locked list (actually this is BUG
   as we have not written the buffer yet)
                                        journal_unmap_buffer() happens

  Maybe what you could verify is, that we are moving the buffer added to
wbuf[] to BJ_Locked list in the write_out_data: loop.
  If it really happens, what I've described above, the fix should be
different. We shouldn't have moved the buffer to BJ_Locked list...

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