linux-kernel - Re: [PATCH block#for-2.6.31] block: add request clone interface

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Date:	Wed, 10 Jun 2009 11:15:29 +0900
From:	Kiyoshi Ueda <k-ueda@...jp.nec.com>
To:	Jens Axboe <jens.axboe@...cle.com>
CC:	linux-kernel@...r.kernel.org,
	device-mapper development <dm-devel@...hat.com>,
	"Jun'ichi Nomura" <j-nomura@...jp.nec.com>,
	Boaz Harrosh <bharrosh@...asas.com>, k-ueda@...jp.nec.com
Subject: Re: [PATCH block#for-2.6.31] block: add request clone interface

Hi Jens,

On 06/10/2009 03:03 AM +0900, Jens Axboe wrote:
> On Tue, Jun 09 2009, Kiyoshi Ueda wrote:
>> Hi Jens,
>>
>> +/*
>> + * Copy request information of the original request to the clone request.
>> + */
>> +static void __blk_rq_prep_clone(struct request *dst, struct request *src)
>> +{
>> +	dst->cpu = src->cpu;
>> +	dst->cmd_flags = (rq_data_dir(src) | REQ_NOMERGE);
>> +	dst->cmd_type = src->cmd_type;
>> +	dst->__sector = blk_rq_pos(src);
>> +	dst->__data_len = blk_rq_bytes(src);
>> +	dst->nr_phys_segments = src->nr_phys_segments;
>> +	dst->ioprio = src->ioprio;
>> +	dst->buffer = src->buffer;
>> +	dst->cmd_len = src->cmd_len;
>> +	dst->cmd = src->cmd;
> 
> Are you making sure that 'src' always exists while 'dst' is alive?

Yes.
Request-based dm is the owner of 'src' (original) and
it never frees 'src' until the 'dst' (clone) are completed.

I avoided deep-copying __cmd/buffer/sense as it's costly
(additional allocation and memcpy).
And I don't think there are any needs for that.
But if anyone really wants that even with the copying cost,
please speak up.

Thanks,
Kiyoshi Ueda
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