linux-kernel - Re: [RFC PATCH net-next v8 02/14] net: page_pool: create hooks for custom page providers

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Message-ID: <20240508142530.GR4718@ziepe.ca>
Date: Wed, 8 May 2024 11:25:30 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Pavel Begunkov <asml.silence@...il.com>
Cc: Mina Almasry <almasrymina@...gle.com>,
	Christoph Hellwig <hch@...radead.org>, netdev@...r.kernel.org,
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Subject: Re: [RFC PATCH net-next v8 02/14] net: page_pool: create hooks for
 custom page providers

On Wed, May 08, 2024 at 12:30:07PM +0100, Pavel Begunkov wrote:

> > I'm not going to pretend to know about page pool details, but dmabuf
> > is the way to get the bulk of pages into a pool within the net stack's
> > allocator and keep that bulk properly refcounted while.> An object like
> > dmabuf is needed for the general case because there are
> > not going to be per-page references or otherwise available.
> 
> They are already pinned, memory is owned by the provider, io_uring
> in this case, and it should not be freed circumventing io_uring,
> and at this stage calling release_pages() is not such a hassle,
> especially comparing to introducing an additional object.

Something needs to co-ordinate when the net stack's allocator is done
with the bulk of pages and when io_uring and do the final
put_user_page() to free it. DMABUF is not an unreasonable choice for
this.

> > topic to me, and honestly hacking into the allocator free function
> > seems a bit weird..
> 
> Do you also think that DMA_BUF_IOCTL_SYNC is a weird hack, because
> it "delays free" by pinning the dmabuf object and letting the user
> read memory instead of copying it? I can find many examples

It seems to me the flow you want is for the driver to allocate a page,
put it on a rx ring, process it through the netstack, and deliver it
to io_uring. io_uring would then sit on the allocation until userspace
it done and return it back to the netstack allocator.

Hooking the free of the netstack allocator and then defering it seems
like a weird and indirect way to get there. Why can't io_uring just be
the entity that does the final free and not mess with the logic
allocator?

Jason