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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Date:	Wed, 17 Nov 2010 02:06:59 -0500
From:	Josef Bacik <josef@...hat.com>
To:	Miao Xie <miaox@...fujitsu.com>
Cc:	viro@...iv.linux.org.uk, Josef Bacik <josef@...hat.com>,
	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 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);

before we do anything else and that way btrfs is satisfied since we won't merge
non logically contiguous requests.

So thats a long-winded way of saying NACK, lets not add even more complicated
special crap for dealing with btrfs when we can just do something like the
above.  Thanks,

Josef
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