lkml.org 
[lkml]   [2010]   [Nov]   [17]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 1/3] direct-io: add a hook for the fs to provide its own bio merging check function
On 	wed, 17 Nov 2010 11:55:28 -0500, Chris Mason wrote:
> Excerpts from Josef Bacik's message of 2010-11-17 07:50:11 -0500:
>> 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,
>
> In this case we're adding complexity to the O_DIRECT mapping code, when
> we really should be adding it to the btrfs submit bio hook. It can
> easily break up the bio into smaller units, which will leave us with a
> smaller number of get_blocks calls overall.
>
> I'm working that out now.

Do you mean you are fixing this bug now?

Thanks
Miao


\
 
 \ /
  Last update: 2010-11-18 02:19    [W:0.569 / U:0.008 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site