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 18:11:03 +0800
From:	Miao Xie <miaox@...fujitsu.com>
To:	Josef Bacik <josef@...hat.com>
CC:	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

Hi, Josef

On wed, 17 Nov 2010 04:37:21 -0500, Josef Bacik wrote:
>> 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);
>   }

As far as I know, get_block() can not make sure the IO doesn't span the chunks or
stripes. Maybe we can do this check in get_blocks(). In this way, we needn't change
vfs.

I have written the patch and is testing it now. Up to now, it works well.

Thanks
Miao
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