linux-kernel - Re: [PATCH 03/16] PCI/P2PDMA: Attempt to set map

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Date:   Sun, 2 May 2021 12:58:38 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Logan Gunthorpe <logang@...tatee.com>,
        <linux-kernel@...r.kernel.org>, <linux-nvme@...ts.infradead.org>,
        <linux-block@...r.kernel.org>, <linux-pci@...r.kernel.org>,
        <linux-mm@...ck.org>, <iommu@...ts.linux-foundation.org>
CC:     Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christian König <christian.koenig@....com>,
        Don Dutile <ddutile@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Jakowski Andrzej <andrzej.jakowski@...el.com>,
        Minturn Dave B <dave.b.minturn@...el.com>,
        Jason Ekstrand <jason@...kstrand.net>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Xiong Jianxin <jianxin.xiong@...el.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH 03/16] PCI/P2PDMA: Attempt to set map_type if it has not
 been set

On 4/8/21 10:01 AM, Logan Gunthorpe wrote:
> Attempt to find the mapping type for P2PDMA pages on the first
> DMA map attempt if it has not been done ahead of time.
> 
> Previously, the mapping type was expected to be calculated ahead of
> time, but if pages are to come from userspace then there's no
> way to ensure the path was checked ahead of time.
> 
> Signed-off-by: Logan Gunthorpe <logang@...tatee.com>
> ---
>   drivers/pci/p2pdma.c | 12 +++++++++---
>   1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index 473a08940fbc..2574a062a255 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -825,11 +825,18 @@ EXPORT_SYMBOL_GPL(pci_p2pmem_publish);
>   static enum pci_p2pdma_map_type pci_p2pdma_map_type(struct pci_dev *provider,
>   						    struct pci_dev *client)
>   {
> +	enum pci_p2pdma_map_type ret;
> +
>   	if (!provider->p2pdma)
>   		return PCI_P2PDMA_MAP_NOT_SUPPORTED;
>   
> -	return xa_to_value(xa_load(&provider->p2pdma->map_types,
> -				   map_types_idx(client)));
> +	ret = xa_to_value(xa_load(&provider->p2pdma->map_types,
> +				  map_types_idx(client)));
> +	if (ret != PCI_P2PDMA_MAP_UNKNOWN)
> +		return ret;
> +
> +	return upstream_bridge_distance_warn(provider, client, NULL,
> +					     GFP_ATOMIC);

Returning a "bridge distance" from a "get map type" routine is jarring,
and I think it is because of a pre-existing problem: the above function
is severely misnamed. Let's try renaming it (and the other one) to
approximately:

     upstream_bridge_map_type_warn()
     upstream_bridge_map_type()

...and that should fix that. Well, that, plus tweaking the kernel doc
comments, which are also confused. I think someone started off thinking
about distances through PCIe, but in the end, the routine boils down to
just a few situations that are not distances at all.

Also, the above will read a little better if it is written like this:

	ret = xa_to_value(xa_load(&provider->p2pdma->map_types,
				  map_types_idx(client)));

	if (ret == PCI_P2PDMA_MAP_UNKNOWN)
		ret = upstream_bridge_map_type_warn(provider, client, NULL,
						    GFP_ATOMIC);
	
	return ret;


>   }
>   
>   static int __pci_p2pdma_map_sg(struct pci_p2pdma_pagemap *p2p_pgmap,
> @@ -877,7 +884,6 @@ int pci_p2pdma_map_sg_attrs(struct device *dev, struct scatterlist *sg,
>   	case PCI_P2PDMA_MAP_BUS_ADDR:
>   		return __pci_p2pdma_map_sg(p2p_pgmap, dev, sg, nents);
>   	default:
> -		WARN_ON_ONCE(1);

Why? Or at least, why, in this patch? It looks like an accidental
leftover from something, seeing as how it is not directly related to the
patch, and is not mentioned at all.


thanks,
-- 
John Hubbard
NVIDIA

>   		return 0;
>   	}
>   }
>