linux-kernel - Re: [PATCH] osdblk: a Linux block device for OSD objects

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Message-ID: <20090408060835.GL5178@kernel.dk>
Date:	Wed, 8 Apr 2009 08:08:35 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Jeff Garzik <jeff@...zik.org>
Cc:	Boaz Harrosh <bharrosh@...asas.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-scsi@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] osdblk: a Linux block device for OSD objects

On Wed, Apr 08 2009, Jeff Garzik wrote:
> Jens Axboe wrote:
>> On Tue, Apr 07 2009, Jeff Garzik wrote:
>>> Boaz Harrosh wrote:
>>>> On 04/03/2009 12:58 PM, Jeff Garzik wrote:
>>>>> Jens Axboe wrote:
>>>>>> This wont work, GFP_NOIO inside the queue lock. You are also only
>>>>>> cloning the front bio, what happens if you have > 1 bio on the request?
>>>>>> You seem to dequeue the request and complete all of it, regardless of
>>>>>> whether bio->bi_size == blk_rq_bytes(rq). I'm assuming you have to clone
>>>>>> because of how the osd_req_{read,write} works, so I'd suggest storing
>>>>>> the byte size in your osdblk_request and only completing that in
>>>>>> osdblk_end_request(). Then do a rq_for_each_bio() look in there, and
>>>>>> only dequeue if you manage to start an osd request for each of them,
>>>>>> THEN moving on to the next request.
>>>> There is nothing preventing from issuing a linked bio list. The only thing
>>>> is that osd_read/write looks at the first bio for total size.
>>>> If the first bio->bi_size does not specify the full length of the chain
>>>> then we should add another parameter to osd_read/write for that.
>>>>
>>>> The original idea was to specifically allow chained bios.
>>>>
>>>> Please advise?
>>> As passed to us from the block layer, there is nothing special about 
>>> the  size of the first bio, AFAIK.
>>>
>>> This seems like a libosd bug?  If you want to support chained bio's, 
>>> I  would presume you would either walk the list and sum all sizes, or 
>>> in  some other way input the total request size?
>>
>> Completely agree, if you want to support passing in a chain, you better
>> make the first bio just part of the chain (not some header bio).
>>
>> And Jeff, you still have that bio_clone() bug in your v2 posting.
>
> Yep, that was noted in the patch description as "The major remaining  
> pre-merge FIXME" :)

Indeed it is, sorry for missing that :-)

-- 
Jens Axboe

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