Re: [RFC PATCH 00/28] Removing struct page from P2PDMA

From: Jason Gunthorpe
Date: Wed Jun 26 2019 - 17:00:22 EST

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