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    SubjectRe: [RFC PATCH 00/28] Removing struct page from P2PDMA
    On Wed, Jun 26, 2019 at 02:45:38PM -0600, Logan Gunthorpe wrote:
    >
    >
    > On 2019-06-26 2:21 p.m., Jason Gunthorpe wrote:
    > > On Wed, Jun 26, 2019 at 12:31:08PM -0600, Logan Gunthorpe wrote:
    > >>> we have a hole behind len where we could store flag. Preferably
    > >>> optionally based on a P2P or other magic memory types config
    > >>> option so that 32-bit systems with 32-bit phys_addr_t actually
    > >>> benefit from the smaller and better packing structure.
    > >>
    > >> That seems sensible. The one thing that's unclear though is how to get
    > >> the PCI Bus address when appropriate. Can we pass that in instead of the
    > >> phys_addr with an appropriate flag? Or will we need to pass the actual
    > >> physical address and then, at the map step, the driver has to some how
    > >> lookup the PCI device to figure out the bus offset?
    > >
    > > I agree with CH, if we go down this path it is a layering violation
    > > for the thing injecting bio's into the block stack to know what struct
    > > device they egress&dma map on just to be able to do the dma_map up
    > > front.
    >
    > Not sure I agree with this statement. The p2pdma code already *must*
    > know and access the pci_dev of the dma device ahead of when it submits
    > the IO to know if it's valid to allocate and use P2P memory at all.

    I don't think we should make drives do that. What if it got CMB memory
    on some other device?

    > > For instance we could use a small hash table of the upper phys addr
    > > bits, or an interval tree, to do the lookup.
    >
    > Yes, if we're going to take a hard stance on this. But using an interval
    > tree (or similar) is a lot more work for the CPU to figure out these
    > mappings that may not be strictly necessary if we could just pass better
    > information down from the submitting driver to the mapping driver.

    Right, this is coming down to an optimization argument. I think there
    are very few cases (Basically yours) where the caller will know this
    info, so we need to support the other cases anyhow.

    I think with some simple caching this will become negligible for cases
    you care about

    Jason

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