Re: [PATCH v2 08/11] blk-mq: Add blk_mq_ops.init_request_no_hctx()

[John Garry]

On 18/08/2021 08:38, Ming Lei wrote:
> On Mon, Aug 09, 2021 at 10:29:35PM +0800, John Garry wrote:
> > Add a variant of the init_request function which does not pass a hctx_idx
> > arg.
> >
> > This is important for shared sbitmap support, as it needs to be ensured for
> > introducing shared static rqs that the LLDD cannot think that requests
> > are associated with a specific HW queue.
> >
> > Signed-off-by: John Garry<john.garry@xxxxxxxxxx>
> > ---
> >   block/blk-mq.c         | 15 ++++++++++-----
> >   include/linux/blk-mq.h |  7 +++++++
> >   2 files changed, 17 insertions(+), 5 deletions(-)
> >
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index f14cc2705f9b..4d6723cfa582 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -2427,13 +2427,15 @@ struct blk_mq_tags *blk_mq_alloc_rq_map(struct blk_mq_tag_set *set,
> >   static int blk_mq_init_request(struct blk_mq_tag_set *set, struct request *rq,
> >   			       unsigned int hctx_idx, int node)
> >   {
> > -	int ret;
> > +	int ret = 0;
> >   
> > -	if (set->ops->init_request) {
> > +	if (set->ops->init_request)
> >   		ret = set->ops->init_request(set, rq, hctx_idx, node);
> > -		if (ret)
> > -			return ret;
> > -	}
> > +	else if (set->ops->init_request_no_hctx)
> > +		ret = set->ops->init_request_no_hctx(set, rq, node);

Hi Ming,

> The only shared sbitmap user of SCSI does not use passed hctx_idx, not
> sure we need such new callback.

Sure, actually most versions of init_request callback don't use 
hctx_idx. Or numa_node arg.
> If you really want to do this, just wondering why not pass '-1' as
> hctx_idx in case of shared sbitmap?

Yeah, I did consider that. hctx_idx is an unsigned, and I generally 
don't like -1U - but that's no big deal. But I also didn't like how it 
relies on the driver init_request callback to check the value, which 
changes the semantics.

Obviously we don't add new versions of init_request for new block 
drivers which use shared sbitmap everyday, and any new ones would get it 
right.

I suppose I can go the way you suggest - I just thought that this method 
was neat as well.

Thanks,
John