Re: [PATCH v4 18/18] PCI/P2PDMA: Log detailed ACS routing diagnostics

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Unconditional debug logging in the high-frequency P2PDMA mapping fast path causes severe log flooding.
--

--- Patch [18]: [PATCH v4 18/18] PCI/P2PDMA: Log detailed ACS routing diagnostics ---
commit 6ad59be5d5d93d88cf70cbce85e9046d3c5b0adb
Author: Leon Romanovsky <leon@kernel.org>

    PCI/P2PDMA: Log detailed ACS routing diagnostics

This commit adds detailed ACS routing diagnostics to assist in identifying
P2PDMA mapping failures. It emits info-level logs covering path details,
ACS controls, and cache state.

> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index e54901d21d8a2..b79eb4ce09f47 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c

[ ... ]

> @@ -1237,23 +1467,38 @@ enum pci_p2pdma_map_type pci_p2pdma_map_type(struct p2pdma_provider *provider,
>  	struct pci_dev *pdev = to_pci_dev(provider->owner);
>  	struct pci_dev *client;
>  	struct pci_p2pdma *p2pdma;
> +	unsigned long cache_index;
> +	bool provider_state;
>  	int dist;
>  
> -	if (!pdev->p2pdma)
> +	if (!pdev->p2pdma) {
> +		pci_info(pdev,
> +			 "P2PDMA ACS debug: map lookup rejected; provider state is absent\n");
>  		return PCI_P2PDMA_MAP_NOT_SUPPORTED;
> +	}
>  
> -	if (!dev_is_pci(dev))
> +	if (!dev_is_pci(dev)) {
> +		dev_info(dev,
> +			 "P2PDMA ACS debug: provider=%s map lookup rejected; client is not PCI\n",
> +			 pci_name(pdev));
>  		return PCI_P2PDMA_MAP_NOT_SUPPORTED;
> +	}
>  
>  	client = to_pci_dev(dev);
> +	cache_index = map_types_idx(client);
>  
>  	rcu_read_lock();
>  	p2pdma = rcu_dereference(pdev->p2pdma);
>  
>  	if (p2pdma)
>  		type = xa_to_value(xa_load(&p2pdma->map_types,
> -					   map_types_idx(client)));
> +					   cache_index));
> +	provider_state = !!p2pdma;
>  	rcu_read_unlock();
> +	pci_info(client,
> +		 "P2PDMA ACS debug: map lookup provider=%s index=%#lx cached=%s(%d) provider-state=%u\n",
> +		 pci_name(pdev), cache_index, pci_p2pdma_map_type_name(type),
> +		 type, provider_state);

[Severity: High]
Does this code risk flooding the kernel log buffer and causing severe performance
degradation?

Since pci_p2pdma_map_type() is invoked on the DMA mapping fast path (for
example, once per P2PDMA I/O request via pci_p2pdma_state() in dma_map_sg()
loops), this unconditional logging will execute on every single P2PDMA
map lookup even when a cached route is successfully used.

Will this saturate the kernel log buffer and cause severe printk/console lock
contention for any P2PDMA-capable block I/O or RDMA operation?

>  
>  	if (type == PCI_P2PDMA_MAP_UNKNOWN)
>  		return calc_map_type_and_dist(pdev, client, &dist, true);

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260821-fix-p2p-acs-v4-0-v4-0-94426b96de73@nvidia.com?part=18