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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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