[PATCH 0/1] nvmet: allow user to set req alloc flag

[Sagi Grimberg]

> Hi,
> 
> When using NVMeOF target in the passthru mode we allocate the request
> with BLK_MQ_REQ_NOWAIT flag. This allocates the request in the following
> manner :-
> 
> nvme_alloc_request()
>   blk_mq_alloc_rquest()
>    blk_mq_queue_enter()
>     if (flag & BLK_MQ_REQ_NOWAIT)
>    	return -EBUSY; <-- return if busy.
> 
> On the NVMe controller which I've the fio random write workload running
> parallel on 32 namespaces with higher queue depth results in I/O error,
> where blk_mq_queue_enter() returning -EBUSY as shown above. This problem
> is not easy to reproduce but occurs once in a while with following error
> (See 1 for detailed log) :-
> 
> test1: (groupid=0, jobs=32): err= 5
> (file:io_u.c:1744, func=io_u error, error=Input/output error):
> 
> When the flag BLK_MQ_REQ_NOWAIT is removed from the allocation the
> workload doen't result in the error.
> 
> This patch fixes the problem with the request allocation by adding
> a new configfs attribute so that user can optionally decide whether
> to use BLK_MQ_REQ_NOWAIT or not. We retain the default behavior by
> using BLK_MQ_REQ_NOWAIT when creating the nvmet passthru subsystem.

Why should we ever set REQ_NOWAIT at all? Nothing prevents the
host(s) queue depth from exceeding the controller queue depth...