linux-kernel - Re: [PATCH v6 00/21] Userspace P2PDMA with O

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Date:   Mon, 16 May 2022 22:31:59 +0000
From:   Chaitanya Kulkarni <chaitanyak@...dia.com>
To:     Logan Gunthorpe <logang@...tatee.com>
CC:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>
Subject: Re: [PATCH v6 00/21] Userspace P2PDMA with O_DIRECT NVMe devices

On 4/7/22 08:46, Logan Gunthorpe wrote:
> Hi,
> 
> This patchset continues my work to add userspace P2PDMA access using
> O_DIRECT NVMe devices. This posting contains some minor fixes and a
> rebase onto v5.18-rc1 which contains cleanup from Christoph around
> free_zone_device_page() that helps to enable this patchset. The
> previous posting was here[1].
> 
> The patchset enables userspace P2PDMA by allowing userspace to mmap()
> allocated chunks of the CMB. The resulting VMA can be passed only
> to O_DIRECT IO on NVMe backed files or block devices. A flag is added
> to GUP() in Patch <>, then Patches <> through <> wire this flag up based
> on whether the block queue indicates P2PDMA support. Patches <>
> through <> enable the CMB to be mapped into userspace by mmaping
> the nvme char device.
> 
> This is relatively straightforward, however the one significant
> problem is that, presently, pci_p2pdma_map_sg() requires a homogeneous
> SGL with all P2PDMA pages or all regular pages. Enhancing GUP to
> support enforcing this rule would require a huge hack that I don't
> expect would be all that pallatable. So the first 13 patches add
> support for P2PDMA pages to dma_map_sg[table]() to the dma-direct
> and dma-iommu implementations. Thus systems without an IOMMU plus
> Intel and AMD IOMMUs are supported. (Other IOMMU implementations would
> then be unsupported, notably ARM and PowerPC but support would be added
> when they convert to dma-iommu).
> 
> dma_map_sgtable() is preferred when dealing with P2PDMA memory as it
> will return -EREMOTEIO when the DMA device cannot map specific P2PDMA
> pages based on the existing rules in calc_map_type_and_dist().
> 
> The other issue is dma_unmap_sg() needs a flag to determine whether a
> given dma_addr_t was mapped regularly or as a PCI bus address. To allow
> this, a third flag is added to the page_link field in struct
> scatterlist. This effectively means support for P2PDMA will now depend
> on CONFIG_64BIT.
> 
> Feedback welcome.
> 
> This series is based on v5.18-rc1. A git branch is available here:
> 
>    https://github.com/sbates130272/linux-p2pmem/  p2pdma_user_cmb_v6
> 
> Thanks,
> 
> Logan
> 
> [1] lkml.kernel.org/r/20220128002614.6136-1-logang@...tatee.com
> 
> --


Do you have any plans to re-spin this ?

-ck