linux-kernel - Re: [RFC PATCH v2 04/11] PCI/P2PDMA: Introduce pci_p2pdma_should_map_bus() and pci_p2pdma_bus

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Date:   Mon, 15 Mar 2021 10:27:08 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Ira Weiny <ira.weiny@...el.com>
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>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christian König <christian.koenig@....com>,
        Ira Weiny <iweiny@...el.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>
Subject: Re: [RFC PATCH v2 04/11] PCI/P2PDMA: Introduce
 pci_p2pdma_should_map_bus() and pci_p2pdma_bus_offset()



On 2021-03-12 6:38 p.m., Ira Weiny wrote:
> On Thu, Mar 11, 2021 at 04:31:34PM -0700, Logan Gunthorpe wrote:
>> Introduce pci_p2pdma_should_map_bus() which is meant to be called by
>> DMA map functions to determine how to map a given p2pdma page.
>>
>> pci_p2pdma_bus_offset() is also added to allow callers to get the bus
>> offset if they need to map the bus address.
>>
>> Signed-off-by: Logan Gunthorpe <logang@...tatee.com>
>> ---
>>  drivers/pci/p2pdma.c       | 50 ++++++++++++++++++++++++++++++++++++++
>>  include/linux/pci-p2pdma.h | 11 +++++++++
>>  2 files changed, 61 insertions(+)
>>
>> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
>> index 7f6836732bce..66d16b7eb668 100644
>> --- a/drivers/pci/p2pdma.c
>> +++ b/drivers/pci/p2pdma.c
>> @@ -912,6 +912,56 @@ void pci_p2pdma_unmap_sg_attrs(struct device *dev, struct scatterlist *sg,
>>  }
>>  EXPORT_SYMBOL_GPL(pci_p2pdma_unmap_sg_attrs);
>>  
>> +/**
>> + * pci_p2pdma_bus_offset - returns the bus offset for a given page
>> + * @page: page to get the offset for
>> + *
>> + * Must be passed a PCI p2pdma page.
>> + */
>> +u64 pci_p2pdma_bus_offset(struct page *page)
>> +{
>> +	struct pci_p2pdma_pagemap *p2p_pgmap = to_p2p_pgmap(page->pgmap);
>> +
>> +	WARN_ON(!is_pci_p2pdma_page(page));
> 
> Shouldn't this check be before the to_p2p_pgmap() call?  

The to_p2p_pgmap() call is just doing pointer arithmetic, so strictly
speaking it doesn't need to be before. We just can't access p2p_pgmap
until it has been checked.

> And I've been told not
> to introduce WARN_ON's.  Should this be?
> 
> 	if (!is_pci_p2pdma_page(page))
> 		return -1;

In this case the WARN_ON is just to guard against misuse of the
function. It should never happen unless a developer changes the code in
a way that is incorrect. So I think that's the correct use of WARN_ON.
Though I might change it to WARN and return, that seems safer.

Logan