Date: Wed, 8 May 2024 17:34:32 +0200
From: Daniel Vetter <daniel@...ll.ch>
To: Pavel Begunkov <asml.silence@...il.com>
Cc: Jason Gunthorpe <jgg@...pe.ca>, Mina Almasry <almasrymina@...gle.com>,
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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 12:35:52PM +0100, Pavel Begunkov wrote:
> On 5/8/24 08:16, Daniel Vetter wrote:
> > 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
> 
> It's not, neither I said it is, but it is an argument against removing
> the network's page pool ops.
> 
> > 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).
> 
> Because in this case it solves nothing and helps with nothing, quite
> the opposite. Just as well we can ask why NVMe doesn't wrap user pages
> into a dmabuf while doing IO.

Because the rules around memory reclaim, gfp nesting and guaranteed
forward progress don't match up for block i/o. I looked quite a bit into
gluing direct i/o into dma-buf because there's vulkan extensions for that,
and it's an absolute mess.
-Sima

> 
> > But these are all internals of the dma-buf exporter, the dma-buf api users
> > don't ever need to care.
> > -Sima
> 
> -- 
> Pavel Begunkov

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

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