[RFC PATCHv2 2/3] nvme: introduce multipath_head_always module param

[Hannes Reinecke]

On 4/25/25 12:33, Nilay Shroff wrote:
> Currently, a multipath head disk node is not created for single-ported
> NVMe adapters or private namespaces. However, creating a head node in
> these cases can help transparently handle transient PCIe link failures.
> Without a head node, features like delayed removal cannot be leveraged,
> making it difficult to tolerate such link failures. To address this,
> this commit introduces nvme_core module parameter multipath_head_always.
> 
> When this param is set to true, it forces the creation of a multipath
> head node regardless NVMe disk or namespace type. So this option allows
> the use of delayed removal of head node functionality even for single-
> ported NVMe disks and private namespaces and thus helps transparently
> handling transient PCIe link failures.
> 
> By default multipath_head_always is set to false, thus preserving the
> existing behavior. Setting it to true enables improved fault tolerance
> in PCIe setups. Moreover, please note that enabling this option would
> also implicitly enable nvme_core.multipath.
> 
> Signed-off-by: Nilay Shroff <nilay at linux.ibm.com>
> ---
>   drivers/nvme/host/multipath.c | 70 +++++++++++++++++++++++++++++++----
>   1 file changed, 63 insertions(+), 7 deletions(-)
> 
I really would model this according to dm-multipath where we have the
'fail_if_no_path' flag.
This can be set for PCIe devices to retain the current behaviour
(which we need for things like 'md' on top of NVMe) whenever the
this flag is set.

And it might be an idea to rename this flag to 'multipath_always_on',
so 'multipath_head_always' might be confusing for people not familiar
with the internal layout of the nvme multipath driver.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
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