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    SubjectRe: [PATCH block#for-2.6.31] block: add request clone interface
    Hi Jens,

    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?
    >> 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.
    >
    > 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:
    http://marc.info/?l=dm-devel&m=124468991432260&w=2

    Thanks,
    Kiyoshi Ueda


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