linux-ext4 - Re: [RFC][PATCH] set_page_buffer_dirty should skip unmapped buffers

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Date:	Mon, 11 Sep 2006 13:45:28 -0700
From:	Badari Pulavarty <pbadari@...ibm.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Andrew Morton <akpm@...l.org>, 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 Mon, 2006-09-11 at 11:46 +0200, Jan Kara wrote:
...
> > 
> > I don't have any performance tests handy. We have some automated tests I 
> > can schedule to run to verify the stability aspects.
>   OK. I've run IOZONE rewrite throughput test on my computer with
> iozone -t 10 -i 0 -s 10M -e
>   2.6.18-rc6 and the same kernel + my patch seem to give almost the same
> results. The strange thing was that both in vanilla and patched kernel there
> were several runs where a write througput (when iozone was creating the file)
> was suddenly 10% of the usual value (18MB/s vs. 2MB/s). The rewrite numbers
> were always fine. Maybe that has something to do with block allocation
> code. Anyway, it is not a regression of my patch so unless your test
> finds some problem I think the patch should be ready for inclusion into
> -mm...

Your patch seems to be working fine. I haven't found any major
regression yet. 

I spent lot of time trying to reproduce the problem with buffer-debug
Andrew sent out - I really wanted to get to bottom of whats really
happening here (since your patch made it go away).

Yes. Your theory is correct. journal_dirty_data() is moving the
buffer-head from commited transaction to current one and
journal_unmap_buffer() is discarding and cleaning up the buffer-head.
Later set_page_dirty() dirties the buffer-head there by causing
BUG() in submit_bh().

Here is the buffer-trace-debug output to confirm it. I can sleep better
now :) Now we can figure out, if your fix is the right one or not ..

Thanks,
Badari

View attachment "buffer-trace.out" of type "text/plain" (13544 bytes)