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

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Date:	Thu, 26 Mar 2009 11:35:29 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jan Kara <jack@...e.cz>
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)


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).


> +	} 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.


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