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Date:   Mon, 19 Jun 2023 11:07:05 -0700
From:   Jakub Kicinski <kuba@kernel.org>
To:     Jesper Dangaard Brouer <jbrouer@redhat.com>
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        Jonathan Lemon <jonathan.lemon@gmail.com>
Subject: Re: Memory providers multiplexing (Was: [PATCH net-next v4 4/5]
 page_pool: remove PP_FLAG_PAGE_FRAG flag)
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On Fri, 16 Jun 2023 22:42:35 +0200 Jesper Dangaard Brouer wrote:
> > 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!

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..)
		...
}