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

From: Jason Gunthorpe
Date: Fri Jun 28 2019 - 00:57:09 EST

On Thu, Jun 27, 2019 at 10:49:43AM -0600, Logan Gunthorpe wrote:

> > I don't think a GPU/FPGA driver will be involved, this would enter the
> > block layer through the O_DIRECT path or something generic.. This the
> > general flow I was suggesting to Dan earlier
> I would say the O_DIRECT path has to somehow call into the driver
> backing the VMA to get an address to appropriate memory (in some way
> vaguely similar to how we were discussing at LSF/MM)

Maybe, maybe no. For something like VFIO the PTE already has the
correct phys_addr_t and we don't need to do anything..

For DEVICE_PRIVATE we need to get the phys_addr_t out - presumably
through a new pagemap op?

> If P2P can't be done at that point, then the provider driver would
> do the copy to system memory, in the most appropriate way, and
> return regular pages for O_DIRECT to submit to the block device.

That only makes sense for the migratable DEVICE_PRIVATE case, it
doesn't help the VFIO-like case, there you'd need to bounce buffer.

> >> I think it would be a larger layering violation to have the NVMe driver
> >> (for example) memcpy data off a GPU's bar during a dma_map step to
> >> support this bouncing. And it's even crazier to expect a DMA transfer to
> >> be setup in the map step.
> >
> > Why? Don't we already expect the DMA mapper to handle bouncing for
> > lots of cases, how is this case different? This is the best place to
> > place it to make it shared.
> This is different because it's special memory where the DMA mapper
> can't possibly know the best way to transfer the data.

Why not? If we have a 'bar info' structure that could have data
transfer op callbacks, infact, I think we might already have similar
callbacks for migrating to/from DEVICE_PRIVATE memory with DMA..

> One could argue that the hook to the GPU/FPGA driver could be in the
> mapping step but then we'd have to do lookups based on an address --
> where as the VMA could more easily have a hook back to whatever driver
> exported it.

The trouble with a VMA hook is that it is only really avaiable when
working with the VA, and it is not actually available during GUP, you
have to have a GUP-like thing such as hmm_range_snapshot that is
specifically VMA based. And it is certainly not available during dma_map.

When working with VMA's/etc it seems there are some good reasons to
drive things off of the PTE content (either via struct page & pgmap or
via phys_addr_t & barmap)

I think the best reason to prefer a uniform phys_addr_t is that it
does give us the option to copy the data to/from CPU memory. That
option goes away as soon as the bio sometimes provides a dma_addr_t.

At least for RDMA, we do have some cases (like siw/rxe, hfi) where
they sometimes need to do that copy. I suspect the block stack is
similar, in the general case.