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Message-ID: <ZjpisrsAZdqTXuLT@phenom.ffwll.local>
Date: Tue, 7 May 2024 19:19:46 +0200
From: Daniel Vetter <daniel@...ll.ch>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: Mina Almasry <almasrymina@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Pavel Begunkov <asml.silence@...il.com>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-alpha@...r.kernel.org, linux-mips@...r.kernel.org,
	linux-parisc@...r.kernel.org, sparclinux@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	bpf@...r.kernel.org, linux-kselftest@...r.kernel.org,
	linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Jonathan Corbet <corbet@....net>,
	Richard Henderson <richard.henderson@...aro.org>,
	Ivan Kokshaysky <ink@...assic.park.msu.ru>,
	Matt Turner <mattst88@...il.com>,
	Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
	"James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
	Helge Deller <deller@....de>, Andreas Larsson <andreas@...sler.com>,
	Jesper Dangaard Brouer <hawk@...nel.org>,
	Ilias Apalodimas <ilias.apalodimas@...aro.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Arnd Bergmann <arnd@...db.de>, Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Andrii Nakryiko <andrii@...nel.org>,
	Martin KaFai Lau <martin.lau@...ux.dev>,
	Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>,
	Yonghong Song <yonghong.song@...ux.dev>,
	John Fastabend <john.fastabend@...il.com>,
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...gle.com>,
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	Steffen Klassert <steffen.klassert@...unet.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	David Ahern <dsahern@...nel.org>,
	Willem de Bruijn <willemdebruijn.kernel@...il.com>,
	Shuah Khan <shuah@...nel.org>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Christian König <christian.koenig@....com>,
	Amritha Nambiar <amritha.nambiar@...el.com>,
	Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
	Alexander Mikhalitsyn <alexander@...alicyn.com>,
	Kaiyuan Zhang <kaiyuanz@...gle.com>,
	Christian Brauner <brauner@...nel.org>,
	Simon Horman <horms@...nel.org>,
	David Howells <dhowells@...hat.com>,
	Florian Westphal <fw@...len.de>,
	Yunsheng Lin <linyunsheng@...wei.com>,
	Kuniyuki Iwashima <kuniyu@...zon.com>, Jens Axboe <axboe@...nel.dk>,
	Arseniy Krasnov <avkrasnov@...utedevices.com>,
	Aleksander Lobakin <aleksander.lobakin@...el.com>,
	Michael Lass <bevan@...co.net>, Jiri Pirko <jiri@...nulli.us>,
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	Lorenzo Bianconi <lorenzo@...nel.org>,
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	Shailend Chand <shailend@...gle.com>,
	Harshitha Ramamurthy <hramamurthy@...gle.com>,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	Jeroen de Borst <jeroendb@...gle.com>,
	Praveen Kaligineedi <pkaligineedi@...gle.com>
Subject: Re: [RFC PATCH net-next v8 02/14] net: page_pool: create hooks for
 custom page providers

On Tue, May 07, 2024 at 01:48:38PM -0300, 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.
> 
> If io_uring wants to take its existing memory pre-registration it can
> wrap that in a dmbauf, and somehow pass it to the netstack. Userspace
> doesn't need to know a dmabuf is being used in the background.

So roughly the current dma-buf design considerations for the users of the
dma-api interfaces:

- It's a memory buffer of fixed length.

- Either that memory is permanently nailed into place with dma_buf_pin
  (and if we add more users than just drm display then we should probably
  figure out the mlock account question for these). For locking hierarchy
  dma_buf_pin uses dma_resv_lock which nests within mmap_sem/vma locks but
  outside of any reclaim/alloc contexts.

- Or the memory is more dynamic, in which case case you need to be able to
  dma_resv_lock when you need the memory and make a promise (as a
  dma_fence) that you'll release the memory within finite time and without
  any further allocations once you've unlocked the dma_buf (because
  dma_fence is in GFP_NORECLAIM). That promise can be waiting for memory
  access to finish, but it can also be a pte invalidate+tlb flush, or some
  kind of preemption, or whatever your hw can do really.

  Also, if you do this dynamic model and need to atomically reserve more
  than one dma_buf, you get to do the wait/wound mutex dance, but that's
  really just a bunch of funny looking error handling code and not really
  impacting the overall design or locking hierarchy.

Everything else we can adjust, but I think the above three are not really
changeable or dma-buf becomes unuseable for gpu drivers.

Note that exporters of dma-buf can pretty much do whatever they feel like,
including rejecting all the generic interfaces/ops, because we also use
dma-buf as userspace handles for some really special memory.
-Sima
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

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