Re: [PATCH v3 4/8] blk-mp: introduce blk_mq_hctx_map_queues

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Tue, Nov 12, 2024 at 05:15:31PM +0100, Daniel Wagner wrote:
> On Tue, Nov 12, 2024 at 04:42:40PM +0100, Greg Kroah-Hartman wrote:
> > On Tue, Nov 12, 2024 at 04:33:09PM +0100, Daniel Wagner wrote:
> > > On Tue, Nov 12, 2024 at 02:58:43PM +0100, Greg Kroah-Hartman wrote:
> > > > > +void blk_mq_hctx_map_queues(struct blk_mq_queue_map *qmap,
> > > > > +			    struct device *dev, unsigned int offset)
> > > > > +
> > > > > +{
> > > > > +	const struct cpumask *mask;
> > > > > +	unsigned int queue, cpu;
> > > > > +
> > > > > +	if (!dev->bus->irq_get_affinity)
> > > > > +		goto fallback;
> > > > 
> > > > I think this is better than hard-coding it, but are you sure that the
> > > > bus will always be bound to the device here so that you have a valid
> > > > bus-> pointer?
> > > 
> > > No, I just assumed the bus pointer is always valid. If it is possible to
> > > have a device without a bus, than I'll better extend the condition to
> > > 
> > > 	if (!dev->bus || !dev->bus->irq_get_affinity)
> > >         	goto fallback;
> > 
> > I don't know if it's possible as I don't know what codepaths are calling
> > this, it was hard to unwind.  But you should check "just to be safe" :)
> 
> The main path to map_queues is via the probe functions. There are some
> more paths like when updating a tagset after the number of queues but
> that is all after the probe function.
> 
> nvme_probe
>   nvme_alloc_admin_tag_set
>     blk_mq_alloc_tag_set
>        blk_mq_update_queue_map
>           set->ops->map_queues
> 	     blk_mq_htcx_map_queues
>   nvme_alloc_io_tag_set
>     blk_mq_alloc_tag_set
>       blk_mq_update_queue_map
>         set->ops->map_queues
>           blk_mq_htcx_map_queues
> 
> virtscsi_probe, hisi_sas_v3_probe, ...
>   scsi_add_host
>     scsi_add_host_with_dma
>       scsi_mq_setup_tags
>          blk_mq_alloc_tag_set
>            blk_mq_update_queue_map
>              set->ops->map_queues
>                blk_mq_htcx_map_queues
> 
> virtblk_probe
>   blk_mq_alloc_tag_set
>     blk_mq_update_queue_map
>       set->ops->map_queues
>         blk_mq_htcx_map_queues
> 
> Does this help?

Ok, that seems fine.  Worst case, you crash and it's obvious that it
needs to be checked in the future :)

thanks,

greg k-h