Memory providers multiplexing (Was: [PATCH net-next v4 4/5] page_pool: remove PP_FLAG_PAGE_FRAG flag)

[Mina Almasry]

On Tue, Jul 11, 2023 at 6:11 AM Jason Gunthorpe <jgg at ziepe.ca> wrote:
>
> On Mon, Jul 10, 2023 at 05:45:05PM -0700, Mina Almasry wrote:
>
> > > At least from my position I want to see MEMORY_DEVICE_PCI_P2PDMA used
> > > to represent P2P memory.
> >
> > Would using p2pdma API instead of dmabuf be an acceptable direction?
>
> "p2pdma API" is really just using MEMORY_DEVICE_PCI_P2PDMA and
> teaching the pagepool how to work with ZONE_DEVICE pages.
>

Yes, that's what I have in mind. Roughly something along the lines
where the device memory provider (GPU or what not) does something like
a pci_p2pdma_add_resource(), and the NIC client driver allocates the
pages from the resource, and if is_pci_p2pdma_page() use
pci_p2pdma_map_sg() instead of dma_map_sg() and whatnot.

> I suspect this will clash badly with Matthew's work here:
>
> https://lore.kernel.org/all/20230111042214.907030-1-willy@infradead.org/
>
> As from a mm side we haven't ever considered that ZONE_DEVICE and
> "netmem" can be composed together. The entire point of netmem like
> stuff is that the allocator hands over the majority of struct page to
> the allocatee, and ZONE_DEVICE can't work like that.
>
> However, assuming that can be solved in some agreeable way then it
> would be OK to go down this path.
>

I think there is potential to solve this in an agreeable way. We may
be able to get netmem ZONE_DEVICE pages using the memdesc idea you
proposed, or something like the xarray meta data you mention below. If
not that, I think Jakub already said that he is considering coming up
with a 'page pool like' API that drivers can use, with an
implementation that is compatible with ZONE_DEVICE pages.

> But, I feel like this is just overall too hard a direction from the mm
> perspective.
>
> I don't know anything about page pool, but the main sticking point is
> its reliance on struct page. If it can find another way to locate its
> meta data (eg an xarray), at least for some cases, it would make
> things alot easier.
>



-- 
Thanks,
Mina