netdev - Re: [PATCH net-next v7 2/9] net: add get_netmem/put

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Message-ID: <87e8bde7-b05e-4a1b-bcef-f6bb3a12315a@redhat.com>
Date: Thu, 13 Mar 2025 11:47:44 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Mina Almasry <almasrymina@...gle.com>, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
 kvm@...r.kernel.org, virtualization@...ts.linux.dev,
 linux-kselftest@...r.kernel.org
Cc: Donald Hunter <donald.hunter@...il.com>, Jakub Kicinski
 <kuba@...nel.org>, "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Simon Horman <horms@...nel.org>,
 Jonathan Corbet <corbet@....net>, Andrew Lunn <andrew+netdev@...n.ch>,
 Jeroen de Borst <jeroendb@...gle.com>,
 Harshitha Ramamurthy <hramamurthy@...gle.com>,
 Kuniyuki Iwashima <kuniyu@...zon.com>, Willem de Bruijn
 <willemb@...gle.com>, David Ahern <dsahern@...nel.org>,
 Neal Cardwell <ncardwell@...gle.com>, Stefan Hajnoczi <stefanha@...hat.com>,
 Stefano Garzarella <sgarzare@...hat.com>, "Michael S. Tsirkin"
 <mst@...hat.com>, Jason Wang <jasowang@...hat.com>,
 Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Eugenio Pérez
 <eperezma@...hat.com>, Shuah Khan <shuah@...nel.org>, sdf@...ichev.me,
 asml.silence@...il.com, dw@...idwei.uk, Jamal Hadi Salim <jhs@...atatu.com>,
 Victor Nogueira <victor@...atatu.com>, Pedro Tammela
 <pctammela@...atatu.com>, Samiullah Khawaja <skhawaja@...gle.com>
Subject: Re: [PATCH net-next v7 2/9] net: add get_netmem/put_netmem support

On 3/8/25 10:40 PM, 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>
> Acked-by: Stanislav Fomichev <sdf@...ichev.me>
> 
> ---
> 
> v5: https://lore.kernel.org/netdev/20250227041209.2031104-2-almasrymina@google.com/
> 
> - Updated to check that the net_iov is devmem before calling
>   net_devmem_put_net_iov().
> 
> - Jakub requested that callers of __skb_frag_ref()/skb_page_unref be
>   inspected to make sure that they generate / anticipate skbs with the
>   correct pp_recycle and unreadable setting:
> 
> skb_page_unref
> ==============
> 
> - callers that are unreachable for unreadable skbs:
> 
> gro_pull_from_frag0, skb_copy_ubufs, __pskb_pull_tail

Why `__pskb_pull_tail` is not reachable? it's called by __pskb_trim(),
via skb_condense().

/P