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

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Date:   Tue, 27 Apr 2021 16:33:51 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Logan Gunthorpe <logang@...tatee.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>,
        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 09/16] dma-direct: Support PCI P2PDMA pages in dma-direct
 map_sg

On Thu, Apr 08, 2021 at 11:01:16AM -0600, Logan Gunthorpe wrote:
> Add PCI P2PDMA support for dma_direct_map_sg() so that it can map
> PCI P2PDMA pages directly without a hack in the callers. This allows
> for heterogeneous SGLs that contain both P2PDMA and regular pages.
> 
> SGL segments that contain PCI bus addresses are marked with
> sg_mark_pci_p2pdma() and are ignored when unmapped.
> 
> Signed-off-by: Logan Gunthorpe <logang@...tatee.com>
>  kernel/dma/direct.c | 25 ++++++++++++++++++++++---
>  1 file changed, 22 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index 002268262c9a..108dfb4ecbd5 100644
> +++ b/kernel/dma/direct.c
> @@ -13,6 +13,7 @@
>  #include <linux/vmalloc.h>
>  #include <linux/set_memory.h>
>  #include <linux/slab.h>
> +#include <linux/pci-p2pdma.h>
>  #include "direct.h"
>  
>  /*
> @@ -387,19 +388,37 @@ void dma_direct_unmap_sg(struct device *dev, struct scatterlist *sgl,
>  	struct scatterlist *sg;
>  	int i;
>  
> -	for_each_sg(sgl, sg, nents, i)
> +	for_each_sg(sgl, sg, nents, i) {
> +		if (sg_is_pci_p2pdma(sg)) {
> +			sg_unmark_pci_p2pdma(sg);

This doesn't seem nice, the DMA layer should only alter the DMA
portion of the SG, not the other portions. Is it necessary?

Jason