linux-kernel - Re: Block regression since 3.1-rc3

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Date:	Sun, 9 Oct 2011 15:16:39 +0800
From:	Shaohua Li <shli@...nel.org>
To:	Mike Snitzer <snitzer@...hat.com>
Cc:	Jeff Moyer <jmoyer@...hat.com>, Jens Axboe <axboe@...nel.dk>,
	Tejun Heo <tj@...nel.org>,
	device-mapper development <dm-devel@...hat.com>,
	Christophe Saout <christophe@...ut.de>,
	linux-kernel@...r.kernel.org
Subject: Re: Block regression since 3.1-rc3

2011/10/9 Shaohua Li <shli@...nel.org>:
> 2011/10/9 Mike Snitzer <snitzer@...hat.com>:
>> On Sat, Oct 08 2011 at  7:02am -0400,
>> Shaohua Li <shli@...nel.org> wrote:
>>
>>> Looks the dm request based flush logic is broken.
>>>
>>> saved_make_request_fn
>>>   __make_request
>>>     blk_insert_flush
>>> but blk_insert_flush doesn't put the original request to list, instead, the
>>> q->flush_rq is in list.
>>> then
>>> dm_request_fn
>>>   blk_peek_request
>>>     dm_prep_fn
>>>       clone_rq
>>>   map_request
>>>     blk_insert_cloned_request
>>> so q->flush_rq is cloned, and get dispatched. but we can't clone q->flush_rq
>>> and use it to do flush. map_request even could assign a different blockdev to
>>> the cloned request.
>>
>> You haven't explained why cloning q->flush_rq is broken.  What is the
>> problem with map_request changing the blockdev?  For the purposes of
>> request-based DM the flush machinery has already managed the processing
>> of the flush at the higher level request_queue.
> hmm, looks I overlook the issue. cloned flush_rq has some problems but can
> be fixed.
> 1. it doesn't set requet->bio, request->bio_tail
> 2. REQ_CLONE_MASK should set REQ_FLUSH_SEQ
oh, don't need set REQ_FLUSH_SEQ, the low level queue will set it
anyway. sorry for
the noise. Jeff's patch looks ok then.
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