linux-kernel - Re: ext3 IO latency measurements (was: Linux 2.6.29)

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Date:	Fri, 27 Mar 2009 22:26:45 +0100
From:	Jan Kara <jack@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Theodore Tso <tytso@....edu>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Arjan van de Ven <arjan@...radead.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Nick Piggin <npiggin@...e.de>,
	Jens Axboe <jens.axboe@...cle.com>,
	David Rees <drees76@...il.com>, Jesper Krogh <jesper@...gh.cc>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Roland McGrath <roland@...hat.com>
Subject: Re: ext3 IO latency measurements (was: Linux 2.6.29)

On Thu 26-03-09 11:35:29, Andrew Morton wrote:
> 
> The patch looks OK to me.
> 
> On Thu, 26 Mar 2009 19:11:06 +0100 Jan Kara <jack@...e.cz> wrote:
> 
> > @@ -1490,6 +1494,16 @@ static int ext3_ordered_writepage(struct page *page,
> >  	if (ext3_journal_current_handle())
> >  		goto out_fail;
> >  
> > +	if (!page_has_buffers(page)) {
> > +		create_empty_buffers(page, inode->i_sb->s_blocksize,
> > +				(1 << BH_Dirty)|(1 << BH_Uptodate));
> 
> This will attach dirty buffers to a clean page, which is an invalid
> state (but OK if we immediately fix it up).
  Yes - actually the page has been dirty just the moment before when we run
clear_page_dirty_for_io() - and at this function could have also created
the clean page with dirty buffers...

> > +	} else if (!walk_page_buffers(NULL, page_buffers(page), 0, PAGE_CACHE_SIZE, NULL, buffer_unmapped)) {
> > +		/* Provide NULL instead of get_block so that we catch bugs if buffers weren't really mapped */
> > +		return block_write_full_page(page, NULL, wbc);
> > +	}
> > +	page_bufs = page_buffers(page);
> > +
> > +
> >  	handle = ext3_journal_start(inode, ext3_writepage_trans_blocks(inode));
> >  
> >  	if (IS_ERR(handle)) {
> 
> And if this error happens we'll go on to run
> redirty_page_for_writepage() which will do the right thing.
> 
> However if PageMappedToDisk() is working right, we should be able to
> avoid that newly-added buffer walk.  Possibly SetPageMappedToDisk()
> isn't being run in all the right places though, dunno.
  Yes, SetPageMappedToDisk is set only by block_read_full_page(),
mpage_readpage() and nobh_write_begin(). Obviously not enough... It would
be nice to improve that but that's another story...

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