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    SubjectRe: [PATCH-v2 2/2] Initialize mempool and elevator only for request-based dm devices
    Hi Nikanth,

    On 08/11/2009 06:05 PM +0900, Nikanth Karthikesan wrote:
    > On Tuesday 11 August 2009 13:36:24 Kiyoshi Ueda wrote:
    >> On 08/10/2009 07:48 PM +0900, Nikanth Karthikesan wrote:
    >>> +
    >>> + /*
    >>> + * reinitialize make_request_fn as it was reset to the
    >>> + * default __make_request by blk_init_allocate_queue
    >>> + */
    >>> + md->saved_make_request_fn = md->queue->make_request_fn;
    >>> + blk_queue_make_request(md->queue, dm_request);
    >>> +
    >>> + blk_queue_softirq_done(md->queue, dm_softirq_done);
    >>> + blk_queue_prep_rq(md->queue, dm_prep_fn);
    >>> + blk_queue_lld_busy(md->queue, dm_lld_busy);
    >>> + }
    >>> +
    >>> __unbind(md);
    >>> r = __bind(md, table, &limits);
    >> The queue has been registered at the device creation time by
    >> add_disk() in alloc_dev().
    >> Since the queue is reconfigured (elevator is attached), you have to
    >> update the queue registration (e.g. unregister, then re-register).
    >> But it may not be easy. At least, there is no exported interface to
    >> unregister/re-register queue.
    >
    > Ah, yes. The scheduler attributes will not be exported in
    > /sys/block/dm*/queue/iosched. Exporting elv_register_queue() and calling it
    > here solves it. Something like..
    >
    > @@ -2203,6 +2199,29 @@ int dm_swap_table(struct mapped_device *md, struct
    > dm_table *table)
    > goto out;
    > }
    >
    > + /* new device is being marked as request-based */
    > + if (!md->map && dm_table_request_based(table)) {
    > + /* initialize queue for request-based dm */
    > + r = blk_init_allocated_queue(md->queue, dm_request_fn, NULL);
    > + if (r)
    > + goto out;
    > +
    > + r = elv_register_queue(md->queue);
    > + /* if (r)
    > + * goto out; Better to ignore, just like add_disk does ;-)
    > + */
    > + /*
    > + * reinitialize make_request_fn as it was reset to the
    > + * default __make_request by blk_init_allocate_queue
    > + */
    > + md->saved_make_request_fn = md->queue->make_request_fn;
    > + blk_queue_make_request(md->queue, dm_request);
    > +
    > + blk_queue_softirq_done(md->queue, dm_softirq_done);
    > + blk_queue_prep_rq(md->queue, dm_prep_fn);
    > + blk_queue_lld_busy(md->queue, dm_lld_busy);
    > + }
    > +
    > __unbind(md);
    > r = __bind(md, table, &limits);
    >
    > I would post the v3 of the patches with this change. Do you see any problems
    > in this?

    Humm, it might work for now, but I disagree with that.

    Since elevator is block internal and dm doesn't really care
    (its initialization is actually hidden in blk_init_allocated_queue()),
    directly calling elv_register_queue() from dm seems not right.
    It will likely introduce a bug by future changes in block layer.

    I think the right approach is to define a proper block layer interface
    to reflect the queue configuration change.
    That's why I said "Updating the queue registration may not be easy".

    Thanks,
    Kiyoshi Ueda



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