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

[Josef Bacik]

On Wed, Nov 17, 2010 at 06:11:03PM +0800, Miao Xie wrote:
> 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.
>

Right thats the idea, if we can't span chunks/stripes we should be doing that
limiting in our get_blocks call and that way we don't have to screw with the
generic direct io stuff too much.  Thanks,

Josef
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