Re: [PATCH block#for-2.6.31] block: add request clone interface
From: Kiyoshi Ueda
Date: Wed Jun 10 2009 - 23:18:08 EST
On 06/10/2009 01:30 PM +0900, Jens Axboe wrote:
> On Wed, Jun 10 2009, Kiyoshi Ueda wrote:
>> 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?
>> 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.
> I just worry that the interface is easy to misuse. You don't document
> the requirement that the src request may not go away while dst is used,
> yet it's an important fact. The function advertises itself as a copy,
> you would not normally expect any such restrictions.
OK, I see.
Since forcing such restrictions by code-level is pretty difficult
(e.g. bio also points pages which are not copied), I'd like to put
documents for this.
Please see the updated patch also reflecting Boaz's comments:
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