linux-kernel - Re: [PATCH 09/16] dma-direct: Support PCI P2PDMA pages in dma-direct map

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Date:   Mon, 3 May 2021 11:04:18 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     John Hubbard <jhubbard@...dia.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 09/16] dma-direct: Support PCI P2PDMA pages in dma-direct
 map_sg

Oops missed a comment:

On 2021-05-02 5:28 p.m., John Hubbard wrote:
>>   int dma_direct_map_sg(struct device *dev, struct scatterlist *sgl, int nents,
>>   		enum dma_data_direction dir, unsigned long attrs)
>>   {
>> -	int i;
>> +	struct pci_p2pdma_map_state p2pdma_state = {};
> 
> Is it worth putting this stuff on the stack--is there a noticeable
> performance improvement from caching the state? Because if it's
> invisible, then simplicity is better. I suspect you're right, and that
> it *is* worth it, but it's good to know for real.

I haven't measured it (it would be hard to measure), but I think it's
fairly clear here. Without the state, xa_load() would need to be called
on *every* page in an SGL that maps only P2PDMA memory from one device.
With the state, it only needs to be called once. xa_load() is cheap, but
it is not that cheap.

There's essentially the same optimization in get_user_pages for
ZONE_DEVICE pages. So, if it is necessary there, it should be necessary
here.

Logan