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

[Ming Lei <ming.lei@redhat.com>]

On Mon, Oct 24, 2022 at 05:56:15PM +0100, John Garry wrote:
> On 24/10/2022 14:27, Ming Lei wrote:
> > > > > -	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?
> > blk_mq_bio_to_request() is one which setup these fields, then you add
> > another one in blk_mq_rq_ctx_init().
> 
> ok, understood.
> 
> > 
> > > 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,
> > Did you know the exact issue on nvme-tcp, nvme-rdma or nvme-fc maybe
> > with blk_mq_alloc_request_hctx()?
> 
> I thought that the original issue was an OoO bounds issue, fixed in
> 14dc7a18. Now there is still some issue in the following link, which is
> still unresolved as I understand:
> https://lore.kernel.org/linux-block/5bd886f1-a7c6-b765-da29-777be0328bc2@grimberg.me/#t
> 
> But I think that 14dc7a18 may still leave undesirable scenario:
> - all cpus in HW queue cpumask may go offline after cpu_online_mask read in
> blk_mq_alloc_request_hctx() and before we get the driver tag and set
> rq->hctx

Yeah.

> 
> > 
> > > 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)?
> > But all cpus on this hctx->cpumask could become offline.
> 
> If all hctx->cpumask are offline then we should not allocate a request and
> this is acceptable. Maybe I am missing your point.

As you saw, this API has the above problem too, but any one of CPUs
may become online later, maybe just during blk_mq_alloc_request_hctx(),
and it is easy to cause inconsistence.

You didn't share your use case, but for nvme connection request, if it
is 1:1 mapping, if any one of CPU becomes offline, the controller
initialization could be failed, that isn't good from user viewpoint at
all.


Thanks,
Ming