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Date: Wed, 8 May 2024 17:34:32 +0200
From: Daniel Vetter <daniel@ffwll.ch>
To: Pavel Begunkov <asml.silence@gmail.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>, Mina Almasry <almasrymina@google.com>,
	Christoph Hellwig <hch@infradead.org>, netdev@vger.kernel.org,
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	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>, Stanislav Fomichev <sdf@google.com>,
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	Lorenzo Bianconi <lorenzo@kernel.org>,
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	Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
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	Jeroen de Borst <jeroendb@google.com>,
	Praveen Kaligineedi <pkaligineedi@google.com>
Subject: Re: [RFC PATCH net-next v8 02/14] net: page_pool: create hooks for
 custom page providers
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	Jonathan Corbet <corbet@lwn.net>,
	Richard Henderson <richard.henderson@linaro.org>,
	Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
	Matt Turner <mattst88@gmail.com>,
	Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
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	Helge Deller <deller@gmx.de>, Andreas Larsson <andreas@gaisler.com>,
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	Ilias Apalodimas <ilias.apalodimas@linaro.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
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	Martin KaFai Lau <martin.lau@linux.dev>,
	Eduard Zingerman <eddyz87@gmail.com>, Song Liu <song@kernel.org>,
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	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>, Stanislav Fomichev <sdf@google.com>,
	Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
	Steffen Klassert <steffen.klassert@secunet.com>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	David Ahern <dsahern@kernel.org>,
	Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
	Shuah Khan <shuah@kernel.org>,
	Sumit Semwal <sumit.semwal@linaro.org>,
	Christian =?iso-8859-1?Q?K=F6nig?= <christian.koenig@amd.com>,
	Amritha Nambiar <amritha.nambiar@intel.com>,
	Maciej Fijalkowski <maciej.fijalkowski@intel.com>,
	Alexander Mikhalitsyn <alexander@mihalicyn.com>,
	Kaiyuan Zhang <kaiyuanz@google.com>,
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	Simon Horman <horms@kernel.org>,
	David Howells <dhowells@redhat.com>,
	Florian Westphal <fw@strlen.de>,
	Yunsheng Lin <linyunsheng@huawei.com>,
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	Arseniy Krasnov <avkrasnov@salutedevices.com>,
	Aleksander Lobakin <aleksander.lobakin@intel.com>,
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	Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
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	Shailend Chand <shailend@google.com>,
	Harshitha Ramamurthy <hramamurthy@google.com>,
	Shakeel Butt <shakeel.butt@linux.dev>,
	Jeroen de Borst <jeroendb@google.com>,
	Praveen Kaligineedi <pkaligineedi@google.com>
References: <20240507161857.GA4718@ziepe.ca>
 <ZjpVfPqGNfE5N4bl@infradead.org>
 <CAHS8izPH+sRLSiZ7vbrNtRdHrFEf8XQ61XAyHuxRSL9Jjy8YbQ@mail.gmail.com>
 <20240507164838.GG4718@ziepe.ca>
 <0d5da361-cc7b-46e9-a635-9a7a4c208444@gmail.com>
 <20240507175644.GJ4718@ziepe.ca>
 <6a50d01a-b5b9-4699-9d58-94e5f8f81c13@gmail.com>
 <20240507233247.GK4718@ziepe.ca>
 <Zjsm3vO6rIY_sw5A@phenom.ffwll.local>
 <1e2823db-504b-4829-856f-3f45a45ccada@gmail.com>
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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