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

On Mon, Jun 19, 2023 at 11:07 AM Jakub Kicinski [off-list ref] wrote:

On Fri, 16 Jun 2023 22:42:35 +0200 Jesper Dangaard Brouer wrote:

quoted

quoted

Former is better for huge pages, latter is better for IO mem
(peer-to-peer DMA). I wonder if you have different use case which
requires a different model :(

I want for the network stack SKBs (and XDP) to support different memory
types for the "head" frame and "data-frags". Eric have described this
idea before, that hardware will do header-split, and we/he can get TCP
data part is another page/frag, making it faster for TCP-streams, but
this can be used for much more.

My proposed use-cases involves more that TCP.  We can easily imagine
NVMe protocol header-split, and the data-frag could be a mem_type that
actually belongs to the harddisk (maybe CPU cannot even read this).  The
same scenario goes for GPU memory, which is for the AI use-case.  IIRC
then Jonathan have previously send patches for the GPU use-case.

I really hope we can work in this direction together,

Perfect, that's also the use case I had in mind. The huge page thing
was just a quick thing to implement as a PoC (although useful in its
own right, one day I'll find the time to finish it, sigh).

That said I couldn't convince myself that for a peer-to-peer setup we
have enough space in struct page to store all the information we need.
Or that we'd get a struct page at all, and not just a region of memory
with no struct page * allocated :S

That'd require serious surgery on the page pool's fast paths to work
around.

I haven't dug into the details, tho. If you think we can use page pool
as a frontend for iouring and/or p2p memory that'd be awesome!

Hello Jakub, I'm looking into device memory (peer-to-peer) networking
actually, and I plan to pursue using the page pool as a front end.

Quick description of what I have so far:
current implementation uses device memory with struct pages; I am
putting all those pages in a gen_pool, and we have written an
allocator that allocates pages from the gen_pool. In the driver, we
use this allocator instead of alloc_page() (the driver in question is
gve which currently doesn't use the page pool). When the driver is
done with the p2p page, it simply decrements the refcount on it and
the page is freed back to the gen_pool.

Test results are good; our best results we're able to achieve ~96%
line rate with incoming packets going straight to device memory and
without bouncing the memory to a host buffer (albeit these results are
on our slighly older, production LTS, I need to work on getting
results from linus/master).

I've discussed your page pool frontend idea with our gve owners and
the idea is attractive. In particular it would be good not to insert
much custom code into the driver to support device memory pages or
other page types. I plan on trying to change my approach to match the
page pool provider you have in progress here:
https://github.com/kuba-moo/linux/tree/pp-providers

In particular the main challenge right now seems to be that my device
memory pages are ZONE_DEVICE pages, which can't be inserted to the
page pool as-is due to the union in struct page between the page pool
entries and the ZONE_DEVICE entries. I have some ideas on how to work
around that I'm looking into.

It sounds like you don't have the time at the moment to work on the
page pool provider idea; I plan to try and get my code working with
that model and propose it if it's successful. Let me know if you have
concerns here.

The workaround solution I had in mind would be to create a narrower API
for just data pages. Since we'd need to sprinkle ifs anyway, pull them
up close to the call site. Allowing to switch page pool for a
completely different implementation, like the one Jonathan coded up for
iouring. Basically

$name_alloc_page(queue)
{
        if (queue->pp)
                return page_pool_dev_alloc_pages(queue->pp);
        else if (queue->iouring..)
                ...
}

-- 
Thanks,
Mina

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