On Saturday 08 August 2009 21:51:59 Mike Snitzer wrote:
> On Sat, Aug 08 2009 at 12:56am -0400,
>
> Nikanth Karthikesan <knikanth@xxxxxxx> wrote:
> > Intialize the request_queue and elevator only when the device is marked
> > as a request-based device. This avoids unnecessary creation of mempool
> > for requests. Also we wrongly initialize the elevator even for bio-based
> > devices. As the /sys/block/dm-*/queue/scheduler is exported for
> > device-mapper devices, it is possible to confuse with scheduler options
> > for bio-based devices where scheduler is not at all used.
> >
> > Signed-off-by: Nikanth Karthikesan <knikanth@xxxxxxx>
>
> I like how this clearly splits out request-based DM queue initialization.
>
Thanks.
> More comments below.
>
> > diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> > index 8a311ea..b01dfbe 100644
> > --- a/drivers/md/dm.c
> > +++ b/drivers/md/dm.c
> > @@ -2203,7 +2199,25 @@ int dm_swap_table(struct mapped_device *md, struct
> > dm_table *table) goto out;
> > }
> >
> > - __unbind(md);
> > + if (md->map)
> > + __unbind(md);
> > + else {
> > + /* new device is being marked as either request-based or bio-based */
> > + if (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;
> > + md->saved_make_request_fn = md->queue->make_request_fn;
> > + blk_queue_make_request(md->queue, dm_request);
>
> This blk_queue_make_request() is redundant as it was already established
> in alloc_dev(). The fact that its behavior is now altered as a result
> of the md->saved_make_request_fn assignment doesn't change the fact that
> the queue will be using dm_request(). Am I missing something?
>
blk_init_allocated_queue() would reset it to __make_request(). So we need to
initialize it again. I would add a comment in v2 of the patch.
> Also, I think the code should be cleaned up as follows:
>
> diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> index b01dfbe..1b8aa43 100644
> --- a/drivers/md/dm.c
> +++ b/drivers/md/dm.c
> @@ -2199,25 +2199,18 @@ int dm_swap_table(struct mapped_device *md, struct
> dm_table *table) goto out;
> }
>
> - if (md->map)
> - __unbind(md);
> - else {
> - /* new device is being marked as either request-based or bio-based */
> - if (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;
> - 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);
> -
> - }
> + 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;
> + md->saved_make_request_fn = md->queue->make_request_fn;
> + 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);
>
> out:
I thought of this. But personally, I did not prefer this, as this makes it
clear that we do __unbind() only on not new devices. But, yes it needs more
level of nesting and looks ugly. Changed in v2 of the patch.
Thanks
Nikanth
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