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

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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@xxxxxxxxxxxx/
[2] https://lore.kernel.org/linux-block/CAPcyv4ifGcrdOtUt8qr7pmFhmecGHqGVre9G0RorGczCGVECQQ@xxxxxxxxxxxxxx/

--

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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