linux-kernel - Re: [PATCH v2 06/22] PCI/P2PDMA: Attempt to set map

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Date:   Fri, 14 May 2021 10:15:28 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Christoph Hellwig <hch@....de>
Cc:     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,
        Stephen Bates <sbates@...thlin.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christian König <christian.koenig@....com>,
        John Hubbard <jhubbard@...dia.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 v2 06/22] PCI/P2PDMA: Attempt to set map_type if it has
 not been set



On 2021-05-14 7:49 a.m., Christoph Hellwig wrote:
> On Thu, May 13, 2021 at 04:31:47PM -0600, 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.
>>
>> With this change it's no longer invalid to call pci_p2pdma_map_sg()
>> before the mapping type is calculated so drop the WARN_ON when that
>> is the case.
> 
> Why?

Before this change, the if the mapping type wasn't already calculated
pci_p2pdma_map_sg() would just fail. This was fine for NVMe-of as it
always called pci_p2pdma_distance() ahead of time which calculated the
mapping type, stored it in the xarray and did not proceed if the two
devices could not talk to each other.

This patch makes it so if the mapping type is not already calculated at
dma map time, it does the calculation. This means the dma map operation
can fail if the two devices aren't able to talk to each other.

Logan