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

John Garry
2021-08-18T08:46:36Z

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@huawei.com>
>> ---
>> 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