[dm-devel] [PATCH] dm mpath: switch paths in dm_blk_ioctl() code path

[Hannes Reinecke]

On 4/20/20 10:29 PM, mwilck at suse.com wrote:
> From: Martin Wilck <mwilck at suse.com>
> 
> SCSI LUN passthrough code such as qemu's "scsi-block" device model
> pass every IO to the host via SG_IO ioctls. Currently, dm-multipath
> calls choose_pgpath() only in the block IO code path, not in the
> ioctl code path (unless current_pgpath is NULL). This has the effect
> that no path switching and thus no load balancing is done for
> SCSI-passthrough IO, unless the active path fails.
> 
> Fix this by using the same logic in multipath_prepare_ioctl() as in
> multipath_clone_and_map().
> 
> Note: The allegedly best path selection algorithm, service-time,
> still wouldn't work perfectly, because the io size of the current
> request is always set to 0. Changing that for the IO passthrough
> case would require the ioctl cmd and arg to be passed to dm's
> prepare_ioctl() method.
> ---
>   drivers/md/dm-mpath.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c
> index 58fd137..f9b6a7a 100644
> --- a/drivers/md/dm-mpath.c
> +++ b/drivers/md/dm-mpath.c
> @@ -1916,7 +1916,7 @@ static int multipath_prepare_ioctl(struct dm_target *ti,
>   	int r;
>   
>   	current_pgpath = READ_ONCE(m->current_pgpath);
> -	if (!current_pgpath)
> +	if (!current_pgpath || !test_bit(MPATHF_QUEUE_IO, &m->flags))
>   		current_pgpath = choose_pgpath(m, 0);
>   
>   	if (current_pgpath) {
> 
Well spotted.

Reviewed-by: Hannes Reinecke <hare at suse.de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke            Teamlead Storage & Networking
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