linux-kernel - Re: [PATCH RFC net-next v1 3/5] net: add get_netmem/put

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Message-ID: <CAHS8izMXKYBiTX_jCNcmN+4unBHXT9jF0CkNWBvjia9eP=Z3zA@mail.gmail.com>
Date: Mon, 27 Jan 2025 14:47:06 -0800
From: Mina Almasry <almasrymina@...gle.com>
To: Stanislav Fomichev <stfomichev@...il.com>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-doc@...r.kernel.org, virtualization@...ts.linux.dev, 
	kvm@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	Donald Hunter <donald.hunter@...il.com>, Jonathan Corbet <corbet@....net>, 
	Andrew Lunn <andrew+netdev@...n.ch>, David Ahern <dsahern@...nel.org>, 
	"Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>, 
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Eugenio Pérez <eperezma@...hat.com>, 
	Stefan Hajnoczi <stefanha@...hat.com>, Stefano Garzarella <sgarzare@...hat.com>, Shuah Khan <shuah@...nel.org>, 
	Kaiyuan Zhang <kaiyuanz@...gle.com>, Pavel Begunkov <asml.silence@...il.com>, 
	Willem de Bruijn <willemb@...gle.com>, Samiullah Khawaja <skhawaja@...gle.com>, 
	Stanislav Fomichev <sdf@...ichev.me>, Joe Damato <jdamato@...tly.com>, dw@...idwei.uk
Subject: Re: [PATCH RFC net-next v1 3/5] net: add get_netmem/put_netmem support

On Thu, Dec 26, 2024 at 11:07 AM Stanislav Fomichev
<stfomichev@...il.com> wrote:
>
> On 12/21, Mina Almasry wrote:
> > Currently net_iovs support only pp ref counts, and do not support a
> > page ref equivalent.
> >
> > This is fine for the RX path as net_iovs are used exclusively with the
> > pp and only pp refcounting is needed there. The TX path however does not
> > use pp ref counts, thus, support for get_page/put_page equivalent is
> > needed for netmem.
> >
> > Support get_netmem/put_netmem. Check the type of the netmem before
> > passing it to page or net_iov specific code to obtain a page ref
> > equivalent.
> >
> > For dmabuf net_iovs, we obtain a ref on the underlying binding. This
> > ensures the entire binding doesn't disappear until all the net_iovs have
> > been put_netmem'ed. We do not need to track the refcount of individual
> > dmabuf net_iovs as we don't allocate/free them from a pool similar to
> > what the buddy allocator does for pages.
> >
> > This code is written to be extensible by other net_iov implementers.
> > get_netmem/put_netmem will check the type of the netmem and route it to
> > the correct helper:
> >
> > pages -> [get|put]_page()
> > dmabuf net_iovs -> net_devmem_[get|put]_net_iov()
> > new net_iovs ->       new helpers
> >
> > Signed-off-by: Mina Almasry <almasrymina@...gle.com>
> >
> > ---
> >  include/linux/skbuff_ref.h |  4 ++--
> >  include/net/netmem.h       |  3 +++
> >  net/core/devmem.c          | 10 ++++++++++
> >  net/core/devmem.h          | 11 +++++++++++
> >  net/core/skbuff.c          | 30 ++++++++++++++++++++++++++++++
> >  5 files changed, 56 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/skbuff_ref.h b/include/linux/skbuff_ref.h
> > index 0f3c58007488..9e49372ef1a0 100644
> > --- a/include/linux/skbuff_ref.h
> > +++ b/include/linux/skbuff_ref.h
> > @@ -17,7 +17,7 @@
> >   */
> >  static inline void __skb_frag_ref(skb_frag_t *frag)
> >  {
> > -     get_page(skb_frag_page(frag));
> > +     get_netmem(skb_frag_netmem(frag));
> >  }
> >
> >  /**
> > @@ -40,7 +40,7 @@ static inline void skb_page_unref(netmem_ref netmem, bool recycle)
> >       if (recycle && napi_pp_put_page(netmem))
> >               return;
> >  #endif
>
> [..]
>
> > -     put_page(netmem_to_page(netmem));
> > +     put_netmem(netmem);
>
> I moved the release operation onto a workqueue in my series [1] to avoid
> calling dmabuf detach (which can sleep) from the socket close path
> (which is called with bh disabled). You should probably do something similar,
> see the trace attached below.
>
> 1: https://github.com/fomichev/linux/commit/3b3ad4f36771a376c204727e5a167c4993d4c65a#diff-3c58b866674b2f9beb5ac7349f81566e4df595c25c647710203549589d450f2dR436
>
> (the condition to trigger that is to have an skb in the write queue
> and call close from the userspace)
>

Thanks for catching this indeed. I've also changed the unbinding to
scheduled_work, although I arrived at a slightly different
implementation that is simpler to my eye. I'll upload what I have for
review shortly as RFC.

-- 
Thanks,
Mina