[dm-devel] [PATCH] dm-mpath: return DM_MAPIO_REQUEUE in case of rq allocation failure

[Bart Van Assche]

On Fri, 2017-09-22 at 09:35 +0800, Ming Lei wrote:
> +		/*
> +		 * blk-mq's SCHED_RESTART can cover this requeue, so
> +		 * we needn't to deal with it by DELAY_REQUEUE. More
> +		 * importantly, we have to return DM_MAPIO_REQUEUE
> +		 * so that blk-mq can get the queue busy feedback,
> +		 * otherwise I/O merge can be hurt.
> +		 */
> +		if (q->mq_ops)
> +			return DM_MAPIO_REQUEUE;
> +		else
> +			return DM_MAPIO_DELAY_REQUEUE;
>  	}

This patch is inferior to what I posted because this patch does not avoid
the delay if multiple LUNs are associated with the same SCSI host. Consider
e.g. the following configuration:
* A single SCSI host with two SCSI LUNs associated to that host, e.g. /dev/sda
  and /dev/sdb.
* A dm-mpath instance has been created on top of /dev/sda.
If all tags are in use by requests queued to /dev/sdb, no dm requests are in
progress and a request is submitted against the dm-mpath device then the
blk_get_request(q, GFP_ATOMIC) call will fail. The request will be requeued
and the queue will be rerun after a delay.

My patch does not introduce a delay in this case.

Bart.