Re: [PATCH] blk-mq: Properly init bios from blk_mq_alloc_request_hctx()

John Garry
2022-10-24T10:58:13Z

On 23/10/2022 14:12, Ming Lei wrote:
>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>> index 8070b6c10e8d..260adeb2e455 100644
>> --- a/block/blk-mq.c
>> +++ b/block/blk-mq.c
>> @@ -402,6 +402,10 @@ static struct request *blk_mq_rq_ctx_init(struct blk_mq_alloc_data *data,
>> }
>> }
>>
>> + rq->__data_len = 0;
>> + rq->__sector = (sector_t) -1;
>> + rq->bio = rq->biotail = NULL;
>> +
>> return rq;
>> }
>>
>> @@ -591,9 +595,6 @@ struct request *blk_mq_alloc_request(struct request_queue *q, blk_opf_t opf,
>> if (!rq)
>> goto out_queue_exit;
>> }
>> - rq->__data_len = 0;
>> - rq->__sector = (sector_t) -1;
>> - rq->bio = rq->biotail = NULL;
> This patch looks not good, why do you switch to initialize the three fields
> twice in fast path?

Can you please show me how these are initialized twice?

If there is a real concern with this then we go with my original idea,
which was to copy the init method of blk_mq_alloc_request() (in
blk_mq_alloc_request_hctx())

>
> BTW, we know blk_mq_alloc_request_hctx() has big trouble, so please
> avoid to extend it to other use cases.

Yeah, I know this, but sometimes we just need to allocate for a specific
HW queue...

For my usecase of interest, it should not impact if the cpumask of the
HW queue goes offline after selecting the cpu in
blk_mq_alloc_request_hctx(), so any race is ok ... I think.

However it should be still possible to make blk_mq_alloc_request_hctx()
more robust. How about using something like work_on_cpu_safe() to
allocate and execute the request with blk_mq_alloc_request() on a cpu
associated with the HW queue, such that we know the cpu is online and
stays online until we execute it? Or also extent to
work_on_cpumask_safe() variant, so that we don't need to try all cpus in
the mask (to see if online)?

Thanks,
John