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

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Date:	Wed, 06 Sep 2006 09:43:51 -0700
From:	Badari Pulavarty <pbadari@...ibm.com>
To:	Jan Kara <jack@...e.cz>
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:
> > On Wed, 2006-09-06 at 17:34 +0200, Jan Kara wrote:
> > > > On Wed, 2006-09-06 at 14:47 +0200, Jan Kara wrote:
> > > > 
> > > > > > Andrew, what should we do ? Do you suggest handling this in jbd
> > > > > > itself (like this patch) ?
> > > > >   Actually that part of commit code needs rewrite anyway (and after that
> > > > > rewrite you get rid of ll_rw_block()) because of other problems - the
> > > > > code assumes that whenever buffer is locked, it is being written to disk
> > > > > which is not true... I have some preliminary patches for that but they
> > > > > are not very nice and so far I didn't have enough time to find a nice
> > > > > solution.
> > > > 
> > > > Are you okay with current not-so-elegant fix ? 
> > >   Actually I don't quite understand how it can happen what you describe
> > > (so probably I missed something). How it can happen that some buffers
> > > are unmapped while we are committing them?  journal_unmap_buffers()
> > > checks whether we are not committing truncated buffers and if so, it
> > > does not do anything to such buffers...
> > > 							Bye
> > > 								Honza
> > 
> > Yep. I spent lot of time trying to understand - why they are not
> > getting skipped :(
> > 
> > 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.

- Badari


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