Date: Mon, 1 Apr 2024 12:22:24 -0700
From: Mina Almasry <almasrymina@...gle.com>
To: Christoph Hellwig <hch@...radead.org>, Marc Harvey <marcharvey@...gle.com>, 
	"Cong Wang ." <cong.wang@...edance.com>
Cc: shakeel.butt@...ux.dev, netdev@...r.kernel.org, 
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	Harshitha Ramamurthy <hramamurthy@...gle.com>, Jeroen de Borst <jeroendb@...gle.com>, 
	Praveen Kaligineedi <pkaligineedi@...gle.com>
Subject: Re: [RFC PATCH net-next v6 02/15] net: page_pool: create hooks for
 custom page providers

On Thu, Mar 28, 2024 at 12:31 AM Christoph Hellwig <hch@...radead.org> wrote:
>
> On Tue, Mar 26, 2024 at 01:19:20PM -0700, Mina Almasry wrote:
> >
> > Are you envisioning that dmabuf support would be added to the block
> > layer
>
> Yes.
>
> > (which I understand is part of the VFS and not driver specific),
>
> The block layer isn't really the VFS, it's just another core stack
> like the network stack.
>
> > or as part of the specific storage driver (like nvme for example)? If
> > we can add dmabuf support to the block layer itself that sounds
> > awesome. We may then be able to do devmem TCP on all/most storage
> > devices without having to modify each individual driver.
>
> I suspect we'll still need to touch the drivers to understand it,
> but hopefully all the main infrastructure can live in the block layer.
>
> > In your estimation, is adding dmabuf support to the block layer
> > something technically feasible & acceptable upstream? I notice you
> > suggested it so I'm guessing yes to both, but I thought I'd confirm.
>
> I think so, and I know there has been quite some interest to at least
> pre-register userspace memory so that the iommu overhead can be
> pre-loaded.  It also is a much better interface for Peer to Peer
> transfers than what we currently have.
>

I think this is positively thrilling news for me. I was worried that
adding devmemTCP support to storage devices would involve using a
non-dmabuf standard of buffer sharing like pci_p2pdma_
(drivers/pci/p2pdma.c) and that would require messy changes to
pci_p2pdma_ that would get nacked. Also it would require adding
pci_p2pdma_ support to devmem TCP, which is a can of worms. If adding
dma-buf support to storage devices is feasible and desirable, that's a
much better approach IMO. (a) it will maybe work with devmem TCP
without any changes needed on the netdev side of things and (b)
dma-buf support may be generically useful and a good contribution even
outside of devmem TCP.

I don't have a concrete user for devmem TCP for storage devices but
the use case is very similar to GPU and I imagine the benefits in perf
can be significant in some setups.

Christoph, if you have any hints or rough specific design in mind for
how dma-buf support can be added to the block layer, please do let us
know and we'll follow your hints to investigate. But I don't want to
use up too much of your time. Marc and I can definitely read enough
code to figure out how to do it ourselves :-)

Marc, please review and consider this thread and work, this could be a
good project for you and I. I imagine the work would be:

1. Investigate how to add dma-buf support to the block layer (maybe
write a prototype code, and maybe even test it with devmem TCP).
2. Share a code or no-code proposal with netdev/fs/block layer mailing
list and try to work through concerns/nacks.
3. Finally share RFC through merging etc.

--
Thanks,
Mina

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