Date:	Thu, 13 Dec 2007 14:59:35 -0500
From:	Mark Lord <liml@....ca>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Mark Lord <lkml@....ca>, Matthew Wilcox <matthew@....cx>,
	IDE/ATA development list <linux-ide@...r.kernel.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	linux-scsi <linux-scsi@...r.kernel.org>
Subject: Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

Jens Axboe wrote:
> On Thu, Dec 13 2007, Mark Lord wrote:
>> Jens Axboe wrote:
>>> On Thu, Dec 13 2007, Mark Lord wrote:
>>>> Mark Lord wrote:
>>>>> Jens Axboe wrote:
>>>>>> On Thu, Dec 13 2007, Mark Lord wrote:
>>>>>>> Matthew Wilcox wrote:
>>>>>>>> On Thu, Dec 13, 2007 at 01:48:18PM -0500, Mark Lord wrote:
>>>>>>>>> Problem confirmed.  2.6.23.8 regularly generates segments up to 
>>>>>>>>> 64KB for libata,
>>>>>>>>> but 2.6.24 uses only 4KB segments and a *few* 8KB segments.
>>>>>>>> Just a suspicion ... could this be slab vs slub?  ie check your 
>>>>>>>> configs
>>>>>>>> are the same / similar between the two kernels.
>>>>>>> ..
>>>>>>>
>>>>>>> Mmmm.. a good thought, that one.
>>>>>>> But I just rechecked, and both have CONFIG_SLAB=y
>>>>>>>
>>>>>>> My guess is that something got changed around when Jens
>>>>>>> reworked the block layer for 2.6.24.
>>>>>>> I'm going to dig around in there now.
>>>>>> I didn't rework the block layer for 2.6.24 :-). The core block layer
>>>>>> changes since 2.6.23 are:
>>>>>>
>>>>>> - Support for empty barriers. Not a likely candidate.
>>>>>> - Shared tag queue fixes. Totally unlikely.
>>>>>> - sg chaining support. Not likely.
>>>>>> - The bio changes from Neil. Of the bunch, the most likely suspects in
>>>>>> this area, since it changes some of the code involved with merges and
>>>>>> blk_rq_map_sg().
>>>>>> - Lots of simple stuff, again very unlikely.
>>>>>>
>>>>>> Anyway, it sounds odd for this to be a block layer problem if you do see
>>>>>> occasional segments being merged. So it sounds more like the input data
>>>>>> having changed.
>>>>>>
>>>>>> Why not just bisect it?
>>>>> ..
>>>>>
>>>>> Because the early 2.6.24 series failed to boot on this machine
>>>>> due to bugs in the block layer -- so the code that caused this regression
>>>>> is probably in the stuff from before the kernels became usable here.
>>>> ..
>>>>
>>>> That sounds more harsh than intended --> the earlier 2.6.24 kernels (up to
>>>> the first couple of -rc* ones failed here because of incompatibilities
>>>> between the block/bio changes and libata.
>>>>
>>>> That's better, I think! 
>>> No worries, I didn't pick it up as harsh just as an odd conclusion :-)
>>>
>>> If I were you, I'd just start from the first -rc that booted for you. If
>>> THAT has the bug, then we'll think of something else. If you don't get
>>> anywhere, I can run some tests tomorrow and see if I can reproduce it
>>> here.
>> ..
>>
>> I believe that *anyone* can reproduce it, since it's broken long before
>> the requests ever get to SCSI or libata.  Which also means that *anyone*
>> who wants to can bisect it, as well.
>>
>> I don't do "bisects".
> 
> It was just a suggestion on how to narrow it down, do as you see fit.
> 
>> But I will dig a bit more and see if I can find the culprit.
> 
> Sure, I'll dig around as well.
..

I wonder if it's 9dfa52831e96194b8649613e3131baa2c109f7dc:
     "Merge blk_recount_segments into blk_recalc_rq_segments" ?

That particular commit does some rather innocent code-shuffling,
but also introduces a couple of new "if (nr_hw_segs == 1" conditions
that were not there before.

Okay git experts:  how do I pull out a kernel at the point of this exact commit ?

Thanks!




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