netdev - Re: [RFC PATCH v3 05/12] netdev: netdevice devmem allocator

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Date: Tue, 7 Nov 2023 14:10:52 -0800
From: Mina Almasry <almasrymina@...gle.com>
To: David Ahern <dsahern@...nel.org>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-arch@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
	linaro-mm-sig@...ts.linaro.org, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Jesper Dangaard Brouer <hawk@...nel.org>, Ilias Apalodimas <ilias.apalodimas@...aro.org>, 
	Arnd Bergmann <arnd@...db.de>, Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
	Shuah Khan <shuah@...nel.org>, Sumit Semwal <sumit.semwal@...aro.org>, 
	Christian König <christian.koenig@....com>, 
	Shakeel Butt <shakeelb@...gle.com>, Jeroen de Borst <jeroendb@...gle.com>, 
	Praveen Kaligineedi <pkaligineedi@...gle.com>, Willem de Bruijn <willemb@...gle.com>, 
	Kaiyuan Zhang <kaiyuanz@...gle.com>
Subject: Re: [RFC PATCH v3 05/12] netdev: netdevice devmem allocator

On Mon, Nov 6, 2023 at 3:44 PM David Ahern <dsahern@...nel.org> wrote:
>
> On 11/5/23 7:44 PM, Mina Almasry wrote:
> > diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> > index eeeda849115c..1c351c138a5b 100644
> > --- a/include/linux/netdevice.h
> > +++ b/include/linux/netdevice.h
> > @@ -843,6 +843,9 @@ struct netdev_dmabuf_binding {
> >  };
> >
> >  #ifdef CONFIG_DMA_SHARED_BUFFER
> > +struct page_pool_iov *
> > +netdev_alloc_devmem(struct netdev_dmabuf_binding *binding);
> > +void netdev_free_devmem(struct page_pool_iov *ppiov);
>
> netdev_{alloc,free}_dmabuf?
>

Can do.

> I say that because a dmabuf can be host memory, at least I am not aware
> of a restriction that a dmabuf is device memory.
>

In my limited experience dma-buf is generally device memory, and
that's really its use case. CONFIG_UDMABUF is a driver that mocks
dma-buf with a memfd which I think is used for testing. But I can do
the rename, it's more clear anyway, I think.

On Mon, Nov 6, 2023 at 11:45 PM Yunsheng Lin <linyunsheng@...wei.com> wrote:
>
> On 2023/11/6 10:44, Mina Almasry wrote:
> > +
> > +void netdev_free_devmem(struct page_pool_iov *ppiov)
> > +{
> > +     struct netdev_dmabuf_binding *binding = page_pool_iov_binding(ppiov);
> > +
> > +     refcount_set(&ppiov->refcount, 1);
> > +
> > +     if (gen_pool_has_addr(binding->chunk_pool,
> > +                           page_pool_iov_dma_addr(ppiov), PAGE_SIZE))
>
> When gen_pool_has_addr() returns false, does it mean something has gone
> really wrong here?
>

Yes, good eye. gen_pool_has_addr() should never return false, but then
again, gen_pool_free()  BUG_ON()s if it doesn't find the address,
which is an extremely severe reaction to what can be a minor bug in
the accounting. I prefer to leak rather than crash the machine. It's a
bit of defensive programming that is normally frowned upon, but I feel
like in this case it's maybe warranted due to the very severe reaction
(BUG_ON).

-- 
Thanks,
Mina