Re: [PATCH v6 02/15] net: generalise net_iov chunk owners

[Christoph Hellwig <hch@infradead.org>]

On Thu, Oct 24, 2024 at 03:23:06PM +0100, Pavel Begunkov wrote:
> > That's not what this series does.  It adds the new memory_provider_ops
> > set of hooks, with once implementation for dmabufs, and one for
> > io_uring zero copy.
> 
> First, it's not a _new_ abstraction over a buffer as you called it
> before, the abstraction (net_iov) is already merged.

Umm, it is a new ops vector.

> Second, you mention devmem TCP, and it's not just a page pool with
> "dmabufs", it's a user API to use it and other memory agnostic
> allocation logic. And yes, dmabufs there is the least technically
> important part. Just having a dmabuf handle solves absolutely nothing.

It solves a lot, becaue it provides a proper abstraction.

> > So you are precluding zero copy RX into anything but your magic
> > io_uring buffers, and using an odd abstraction for that.
> 
> Right io_uring zero copy RX API expects transfer to happen into io_uring
> controlled buffers, and that's the entire idea. Buffers that are based
> on an existing network specific abstraction, which are not restricted to
> pages or anything specific in the long run, but the flow of which from
> net stack to user and back is controlled by io_uring. If you worry about
> abuse, io_uring can't even sanely initialise those buffers itself and
> therefore asking the page pool code to do that.

No, I worry about trying to io_uring for not good reason. This
pre-cludes in-kernel uses which would be extremly useful for
network storage drivers, and it precludes device memory of all
kinds.

> I'm even more confused how that would help. The user API has to
> be implemented and adding a new dmabuf gives nothing, not even
> mentioning it's not clear what semantics of that beast is
> supposed to be.
>

The dma-buf maintainers already explained to you last time
that there is absolutely no need to use the dmabuf UAPI, you
can use dma-bufs through in-kernel interfaces just fine.