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

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Date:	Mon, 11 Sep 2006 11:46:42 +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

  Hi,

> >>>Original commit code assumes, that when a buffer on BJ_SyncData list is 
> >>>locked,
> >>>it is being written to disk. But this is not true and hence it can lead 
> >>>to a
> >>>potential data loss on crash. Also the code didn't count with the fact 
> >>>that
> >>>journal_dirty_data() can steal buffers from committing transaction and 
> >>>hence
> >>>could write buffers that no longer belong to the committing transaction.
> >>>Finally it could possibly happen that we tried writing out one buffer 
> >>>several
> >>>times.
> >>>
> >>>The patch below tries to solve these problems by a complete rewrite of 
> >>>the data
> >>>commit code. We go through buffers on t_sync_datalist, lock buffers 
> >>>needing
> >>>write out and store them in an array. Buffers are also immediately 
> >>>refiled to
> >>>BJ_Locked list or unfiled (if the write out is completed). When the 
> >>>array is
> >>>full or we have to block on buffer lock, we submit all accumulated 
> >>>buffers for
> >>>IO.
> >>>
> >>>Signed-off-by: Jan Kara <jack@...e.cz>
> >>>
> >>> 
> >>>      
> >>I have been running 4+ hours with this patch and seems to work fine. I 
> >>haven't hit any
> >>assert yet :)
> >>
> >>I will let it run till tomorrow. I will let you know, how it goes.
> >>    
> >  Great, thanks. BTW: Do you have any performance tests handy? The
> >changes are big enough to cause some unexpected performance regressions,
> >livelocks... If you don't have anything ready, I can setup and run
> >something myself.  Just that I don't like this testing too much ;).
> >  
> Tests are still running fine.
> 
> 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...

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