Re: [RFC PATCH v2 00/11] Add support to dma_map_sg for P2PDMA

[Robin Murphy <robin.murphy@arm.com>]

On 2021-03-11 23:31, Logan Gunthorpe wrote:
> Hi,
> 
> This is a rework of the first half of my RFC for doing P2PDMA in userspace
> with O_DIRECT[1].
> 
> The largest issue with that series was the gross way of flagging P2PDMA
> SGL segments. This RFC proposes a different approach, (suggested by
> Dan Williams[2]) which uses the third bit in the page_link field of the
> SGL.
> 
> This approach is a lot less hacky but comes at the cost of adding a
> CONFIG_64BIT dependency to CONFIG_PCI_P2PDMA and using up the last
> scarce bit in the page_link. For our purposes, a 64BIT restriction is
> acceptable but it's not clear if this is ok for all usecases hoping
> to make use of P2PDMA.
> 
> Matthew Wilcox has already suggested (off-list) that this is the wrong
> approach, preferring a new dma mapping operation and an SGL replacement. I
> don't disagree that something along those lines would be a better long
> term solution, but it involves overcoming a lot of challenges to get
> there. Creating a new mapping operation still means adding support to more
> than 25 dma_map_ops implementations (many of which are on obscure
> architectures) or creating a redundant path to fallback with dma_map_sg()
> for every driver that uses the new operation. This RFC is an approach
> that doesn't require overcoming these blocks.

I don't really follow that argument - you're only adding support to two 
implementations with the awkward flag, so why would using a dedicated 
operation instead be any different? Whatever callers need to do if 
dma_pci_p2pdma_supported() says no, they could equally do if 
dma_map_p2p_sg() (or whatever) returns -ENXIO, no?

We don't try to multiplex .map_resource through .map_page, so there 
doesn't seem to be any good reason to force that complexity on .map_sg 
either. And having a distinct API from the outset should make it a lot 
easier to transition to better "list of P2P memory regions" data 
structures in future without rewriting the whole world. As it is, there 
are potential benefits in a P2P interface which can define its own 
behaviour - for instance if threw out the notion of segment merging it 
could save a load of bother by just maintaining the direct correlation 
between pages and DMA addresses.

Robin.

> Any alternative ideas or feedback is welcome.
> 
> These patches are based on v5.12-rc2 and a git branch is available here:
> 
>    https://github.com/sbates130272/linux-p2pmem/  p2pdma_dma_map_ops_rfc
> 
> A branch with the patches from the previous RFC that add userspace
> O_DIRECT support is available at the same URL with the name
> "p2pdma_dma_map_ops_rfc+user" (however, none of the issues with those
> extra patches from the feedback of the last posting have been fixed).
> 
> Thanks,
> 
> Logan
> 
> [1] https://lore.kernel.org/linux-block/20201106170036.18713-1-logang@deltatee.com/
> [2] https://lore.kernel.org/linux-block/CAPcyv4ifGcrdOtUt8qr7pmFhmecGHqGVre9G0RorGczCGVECQQ@mail.gmail.com/
> 
> --
> 
> Logan Gunthorpe (11):
>    PCI/P2PDMA: Pass gfp_mask flags to upstream_bridge_distance_warn()
>    PCI/P2PDMA: Avoid pci_get_slot() which sleeps
>    PCI/P2PDMA: Attempt to set map_type if it has not been set
>    PCI/P2PDMA: Introduce pci_p2pdma_should_map_bus() and
>      pci_p2pdma_bus_offset()
>    lib/scatterlist: Add flag for indicating P2PDMA segments in an SGL
>    dma-direct: Support PCI P2PDMA pages in dma-direct map_sg
>    dma-mapping: Add flags to dma_map_ops to indicate PCI P2PDMA support
>    iommu/dma: Support PCI P2PDMA pages in dma-iommu map_sg
>    block: Add BLK_STS_P2PDMA
>    nvme-pci: Check DMA ops when indicating support for PCI P2PDMA
>    nvme-pci: Convert to using dma_map_sg for p2pdma pages
> 
>   block/blk-core.c            |  2 +
>   drivers/iommu/dma-iommu.c   | 63 +++++++++++++++++++++-----
>   drivers/nvme/host/core.c    |  3 +-
>   drivers/nvme/host/nvme.h    |  2 +-
>   drivers/nvme/host/pci.c     | 38 +++++++---------
>   drivers/pci/Kconfig         |  2 +-
>   drivers/pci/p2pdma.c        | 89 +++++++++++++++++++++++++++++++------
>   include/linux/blk_types.h   |  7 +++
>   include/linux/dma-map-ops.h |  3 ++
>   include/linux/dma-mapping.h |  5 +++
>   include/linux/pci-p2pdma.h  | 11 +++++
>   include/linux/scatterlist.h | 49 ++++++++++++++++++--
>   kernel/dma/direct.c         | 35 +++++++++++++--
>   kernel/dma/mapping.c        | 21 +++++++--
>   14 files changed, 271 insertions(+), 59 deletions(-)
> 
> 
> base-commit: a38fd8748464831584a19438cbb3082b5a2dab15
> --
> 2.20.1
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