netdev - Re: [PATCH RFC 3/8] memory-provider: dmabuf devmem memory provider

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Date: Mon, 13 Nov 2023 18:05:54 -0500
From: Jakub Kicinski <kuba@...nel.org>
To: Mina Almasry <almasrymina@...gle.com>, Yunsheng Lin
 <linyunsheng@...wei.com>
Cc: davem@...emloft.net, pabeni@...hat.com, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, Willem de Bruijn <willemb@...gle.com>,
 Kaiyuan Zhang <kaiyuanz@...gle.com>, Jesper Dangaard Brouer
 <hawk@...nel.org>, Ilias Apalodimas <ilias.apalodimas@...aro.org>, Eric
 Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH RFC 3/8] memory-provider: dmabuf devmem memory provider

On Mon, 13 Nov 2023 05:42:16 -0800 Mina Almasry wrote:
> You're doing exactly what I think you're doing, and what was nacked in RFC v1.
> 
> You've converted 'struct page_pool_iov' to essentially become a
> duplicate of 'struct page'. Then, you're casting page_pool_iov* into
> struct page* in mp_dmabuf_devmem_alloc_pages(), then, you're calling
> mm APIs like page_ref_*() on the page_pool_iov* because you've fooled
> the mm stack into thinking dma-buf memory is a struct page.
> 
> RFC v1 was almost exactly the same, except instead of creating a
> duplicate definition of struct page, it just allocated 'struct page'
> instead of allocating another struct that is identical to struct page
> and casting it into struct page.
> 
> I don't think what you're doing here reverses the nacks I got in RFC
> v1. You also did not CC any dma-buf or mm people on this proposal that
> would bring up these concerns again.

Right, but the mirror struct has some appeal to a non-mm person like
myself. The problem IIUC is that this patch is the wrong way around, we
should be converting everyone who can deal with non-host mem to struct
page_pool_iov. Using page_address() on ppiov which hns3 seems to do in
this series does not compute for me.

Then we can turn the existing non-iov helpers to be a thin wrapper with
just a cast from struct page to struct page_pool_iov, and a call of the
iov helper. Again - never cast the other way around.

Also I think this conversion can be done completely separately from the
mem provider changes. Just add struct page_pool_iov and start using it.

Does that make more sense?