linux-kernel - Re: [PATCH 1/3] direct-io: add a hook for the fs to provide its own bio merging check function

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Message-ID: <20101117093720.GG5618@dhcp231-156.rdu.redhat.com>
Date:	Wed, 17 Nov 2010 04:37:21 -0500
From:	Josef Bacik <josef@...hat.com>
To:	Josef Bacik <josef@...hat.com>
Cc:	Miao Xie <miaox@...fujitsu.com>, viro@...iv.linux.org.uk,
	Chris Mason <chris.mason@...cle.com>,
	Linux Fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux Btrfs <linux-btrfs@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ito <t-itoh@...fujitsu.com>
Subject: Re: [PATCH 1/3] direct-io: add a hook for the fs to provide its
	own bio merging check function

On Wed, Nov 17, 2010 at 02:06:59AM -0500, Josef Bacik wrote:
> On Wed, Nov 17, 2010 at 12:18:35PM +0800, Miao Xie wrote:
> > BTRFS can not submit bios that span its chunks or stripes, so it needs a
> > function to check it when we want to add a page into the bios. So we add a
> > can_merge_io hook to do it.
> > 
> 
> Heh so I was going to fix this after the hole punching stuff.  The fact is btrfs
> maps everything that is ok to do in one IO via get_blocks().  So all we need to
> do is add another DIO_ flag to tell us to treat each get_blocks() call as
> discrete.  I wanted to use buffer_boundary for this, but I think it's too
> drastic of a change for people who already use buffer_boundary();
> 
> What happens today is that say we map 4k, we do submit_page_section, but if this
> is our first bit of IO we just set dio->cur_page and such and then loop again.
> Say there is 4k-hole-4k, we do the next mapping and set buffer_boundary again,
> and come into submit_page_section and because cur_page is set, we do
> dio_send_cur_page.  Because there is no dio->bio we setup a new bio, but when we
> do that we clear dio->boundary, and leave the bio all setup.  So the next time
> we loop around the tail 4k gets added to our previously setup bio and boom we
> hit this problem with btrfs.
> 
> If we can add a DIO_GET_BLOCKS_DISCRETE or some other such non-sense then we can
> easily kill all the logical offset code I had and just make some simple changes
> to make the DIO stuff work for us.  All we do is in get_more_blocks we do
> 
> if ((dio->flags & DIO_GET_BLOCKS_DISCRETE) && dio->bio)
> 	dio_submit_bio(dio);
>

Right after I went to bed I realized this should be

if (dio->flags & DIO_GET_BLOCKS_DISCRETE) {
	if (dio->cur_page) {
		dio_send_cur_page(dio);
		page_cache_release(dio->cur_page);
		dio->cur_page = NULL;
	}

	if (dio->bio)
		dio_submit_bio(dio);
 }

Thanks,

Josef
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