Re: [PATCH V2 2/2] nvme-multipath: enable PCI P2PDMA for multipath devices

[Christoph Hellwig <hch@lst.de>]

On Wed, Apr 08, 2026 at 12:25:37AM -0700, Chaitanya Kulkarni wrote:
> From: Kiran Kumar Modukuri <kmodukuri@nvidia.com>
> 
> NVMe multipath does not expose BLK_FEAT_PCI_P2PDMA on the head disk
> even when the underlying controller supports it.
> 
> Set BLK_FEAT_PCI_P2PDMA in nvme_mpath_alloc_disk() when the controller
> advertises P2PDMA support via ctrl->ops->supports_pci_p2pdma.
> 
> Since multipath can match paths across different transports (e.g. PCIe
> and FC), not all paths are guaranteed to support P2PDMA. Clear
> BLK_FEAT_PCI_P2PDMA from the head disk in nvme_mpath_add_disk() if the
> newly added path does not support it, ensuring the feature is only
> advertised when every member supports it.
> 
> Signed-off-by: Kiran Kumar Modukuri <kmodukuri@nvidia.com>
> Signed-off-by: Chaitanya Kulkarni <kch@nvidia.com>
> ---
>  drivers/nvme/host/multipath.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
> index ba00f0b72b85..48d920ce803f 100644
> --- a/drivers/nvme/host/multipath.c
> +++ b/drivers/nvme/host/multipath.c
> @@ -737,6 +737,9 @@ int nvme_mpath_alloc_disk(struct nvme_ctrl *ctrl, struct nvme_ns_head *head)
>  		BLK_FEAT_POLL | BLK_FEAT_ATOMIC_WRITES;
>  	if (head->ids.csi == NVME_CSI_ZNS)
>  		lim.features |= BLK_FEAT_ZONED;
> +	if (ctrl->ops && ctrl->ops->supports_pci_p2pdma &&
> +	    ctrl->ops->supports_pci_p2pdma(ctrl))
> +		lim.features |= BLK_FEAT_PCI_P2PDMA;

I think we can just add this unconditionally here, similar to all the
other feaures above the ZNS conditional as any non-supporting controller
will clear it later.

>  
>  void nvme_mpath_add_disk(struct nvme_ns *ns, __le32 anagrpid)
>  {
> +	struct nvme_ns_head *head = ns->head;
> +
> +	/*
> +	 * Clear BLK_FEAT_PCI_P2PDMA on the head disk if this path does not
> +	 * support it. Multipath may span different transports (e.g. PCIe and
> +	 * FC), so every member must support P2PDMA for it to be safe on the
> +	 * head disk.
> +	 */
> +	if (head->disk && !blk_queue_pci_p2pdma(ns->queue)) {
> +		struct queue_limits lim =
> +			queue_limits_start_update(head->disk->queue);
> +		lim.features &= ~BLK_FEAT_PCI_P2PDMA;
> +		queue_limits_commit_update(head->disk->queue, &lim);
> +	}

And this really should go into the core block code in blk_stack_limits,
so that BLK_FEAT_PCI_P2PDMA is cleared whenever a non-confirming
device is added, similar to BLK_FEAT_NOWAIT and BLK_FEAT_POLL.