Message-ID: <Zjsm3vO6rIY_sw5A@phenom.ffwll.local>
Date: Wed, 8 May 2024 09:16:46 +0200
From: Daniel Vetter <daniel@...ll.ch>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: Pavel Begunkov <asml.silence@...il.com>,
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Subject: Re: [RFC PATCH net-next v8 02/14] net: page_pool: create hooks for
 custom page providers

On Tue, May 07, 2024 at 08:32:47PM -0300, Jason Gunthorpe wrote:
> On Tue, May 07, 2024 at 08:35:37PM +0100, Pavel Begunkov wrote:
> > On 5/7/24 18:56, Jason Gunthorpe wrote:
> > > On Tue, May 07, 2024 at 06:25:52PM +0100, Pavel Begunkov wrote:
> > > > On 5/7/24 17:48, Jason Gunthorpe wrote:
> > > > > On Tue, May 07, 2024 at 09:42:05AM -0700, Mina Almasry wrote:
> > > > > 
> > > > > > 1. Align with devmem TCP to use udmabuf for your io_uring memory. I
> > > > > > think in the past you said it's a uapi you don't link but in the face
> > > > > > of this pushback you may want to reconsider.
> > > > > 
> > > > > dmabuf does not force a uapi, you can acquire your pages however you
> > > > > want and wrap them up in a dmabuf. No uapi at all.
> > > > > 
> > > > > The point is that dmabuf already provides ops that do basically what
> > > > > is needed here. We don't need ops calling ops just because dmabuf's
> > > > > ops are not understsood or not perfect. Fixup dmabuf.
> > > > 
> > > > Those ops, for example, are used to efficiently return used buffers
> > > > back to the kernel, which is uapi, I don't see how dmabuf can be
> > > > fixed up to cover it.
> > > 
> > > Sure, but that doesn't mean you can't use dma buf for the other parts
> > > of the flow. The per-page lifetime is a different topic than the
> > > refcounting and access of the entire bulk of memory.
> > 
> > Ok, so if we're leaving uapi (and ops) and keep per page/sub-buffer as
> > is, the rest is resolving uptr -> pages, and passing it to page pool in
> > a convenient to page pool format (net_iov).
> 
> 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.
> 
> What you seem to want is to alter how the actual allocation flow works
> from that bulk of memory and delay the free. It seems like a different
> topic to me, and honestly hacking into the allocator free function
> seems a bit weird..

Also I don't see how it's an argument against dma-buf as the interface for
all these, because e.g. ttm internally does have a page pool because
depending upon allocator, that's indeed beneficial. Other drm drivers have
more buffer-based concepts for opportunistically memory around, usually
by marking buffers that are just kept as cache as purgeable (which is a
concept that goes all the way to opengl/vulkan).

But these are all internals of the dma-buf exporter, the dma-buf api users
don't ever need to care.
-Sima
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

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