Re: [RFC PATCH 3/5] mm/vma: add support for peer to peer to device vma

[Jason Gunthorpe]

On Wed, Jan 30, 2019 at 10:17:27AM -0700, Logan Gunthorpe wrote:
> 
> 
> On 2019-01-29 9:18 p.m., Jason Gunthorpe wrote:
> > Every attempt to give BAR memory to struct page has run into major
> > trouble, IMHO, so I like that this approach avoids that.
> > 
> > And if you don't have struct page then the only kernel object left to
> > hang meta data off is the VMA itself.
> > 
> > It seems very similar to the existing P2P work between in-kernel
> > consumers, just that VMA is now mediating a general user space driven
> > discovery process instead of being hard wired into a driver.
> 
> But the kernel now has P2P bars backed by struct pages and it works
> well. 

I don't think it works that well..

We ended up with a 'sgl' that is not really a sgl, and doesn't work
with many of the common SGL patterns. sg_copy_buffer doesn't work,
dma_map, doesn't work, sg_page doesn't work quite right, etc.

Only nvme and rdma got the special hacks to make them understand these
p2p-sgls, and I'm still not convinced some of the RDMA drivers that
want access to CPU addresses from the SGL (rxe, usnic, hfi, qib) don't
break in this scenario.

Since the SGLs become broken, it pretty much means there is no path to
make GUP work generically, we have to go through and make everything
safe to use with p2p-sgls before allowing GUP. Which, frankly, sounds
impossible with all the competing objections.

But GPU seems to have a problem unrelated to this - what Jerome wants
is to have two faulting domains for VMA's - visible-to-cpu and
visible-to-dma. The new op is essentially faulting the pages into the
visible-to-dma category and leaving them invisible-to-cpu.

So that duality would still have to exists, and I think p2p_map/unmap
is a much simpler implementation than trying to create some kind of
special PTE in the VMA..

At least for RDMA, struct page or not doesn't really matter. 

We can make struct pages for the BAR the same way NVMe does.  GPU is
probably the same, just with more mememory at stake?  

And maybe this should be the first implementation. The p2p_map VMA
operation should return a SGL and the caller should do the existing
pci_p2pdma_map_sg() flow.. 

Worry about optimizing away the struct page overhead later?

Jason