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

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Date:   Tue, 11 May 2021 12:05:49 -0400
From:   Don Dutile <ddutile@...hat.com>
To:     John Hubbard <jhubbard@...dia.com>,
        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>,
        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 5/2/21 3:58 PM, John Hubbard wrote:
> 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.
>
+1. didn't like the 'distance' check  for a 'connection check" in the beginning, and looks like this is the time to clean it out.
:)

> 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,