Re: [PATCH v3] nvme-multipath: add fail_if_no_path sysfs attribute

[Hannes Reinecke <hare@suse.de>]

On 9/23/26 2:49 AM, Krishna Iyer wrote:
> When no usable path exists, I/O on a multipath namespace is queued
> until a path returns. With ctrl_loss_tmo=-1 that can be forever:
> during a long fabric outage any process waiting on the I/O is stuck in
> D state. We hit this on virtualization hosts, where a SIGKILLed VM
> process cannot exit while draining I/O to an unreachable NVMe/TCP
> target.
> 
> Nothing can fail this I/O without tearing something down: controller
> deletion takes every namespace on the controller with it.
> 
> Add a fail_if_no_path attribute on the ns-head disk: a persistent
> per-namespace policy to fail parked and newly arriving I/O instead of
> queueing it when no usable path exists. It is enforced where a path is
> known to be unusable: CONNECTING controllers and LIVE controllers with
> the path ANA inaccessible or persistent-loss stop counting as
> available, RESETTING and ANA change keep queueing, and with no
> controllers left the policy overrides the delayed_removal_secs
> queueing window. Controller state is untouched and reconnects
> continue. Like dm's fail_if_no_path, the policy is transport agnostic.
> 
> Assisted-by: Claude:claude-fable-5
> Signed-off-by: Krishna Iyer <kiyer@crusoe.ai>
> ---
> Changes since v2 [1]:
> - use the nvme_state_is_live() helper for the ANA state check, keeping
>    the explicit NVME_ANA_CHANGE carve-out so transient ANA transitions
>    still queue (Nilay)
> - return early when the stored value matches the current setting,
>    skipping synchronize_srcu() and the requeue kick (Nilay)
> 
[ .. ]
> diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
> index a643c4723a69..9773538101ca 100644
> --- a/drivers/nvme/host/nvme.h
> +++ b/drivers/nvme/host/nvme.h
> @@ -590,6 +590,7 @@ struct nvme_ns_head {
>   #define NVME_NSHEAD_DISK_LIVE		0
>   #define NVME_NSHEAD_QUEUE_IF_NO_PATH	1
>   #define NVME_NSHEAD_CDEV_LIVE		2
> +#define NVME_NSHEAD_FAIL_IF_NO_PATH	3
>   	struct nvme_ns __rcu_guarded	*current_path[];
>   #endif
>   };

Remind me again: what is the meaning of 'NVME_NSHEAD_QUEUE_IF_NO_PATH'?
Isn't this the exact opposite of the proposed
'NVME_NSHEAD_FAIL_IF_NO_PATH'?
IOW can't you rewrite this patch to use the existing 
'NVME_NSHEAD_QUEUE_IF_NO_PATH' flag instead of adding another?
(And if you have to introduce 'FAIL_IF_NO_PATH' you'd have to explain
what exactly should happen if QUEUE_IF_NO_PATH _and_ FAIL_IF_NO_PATH
are set ...)

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@suse.de                                +49 911 74053 688
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