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

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Message-ID: <CAHS8izPbWVXS9gPBbuR4EisT806+3woqD=njDHj8hNJLcDU4DA@mail.gmail.com>
Date: Thu, 13 Mar 2025 08:37:59 -0700
From: Mina Almasry <almasrymina@...gle.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: 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, 
	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>, 
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	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 Thu, Mar 13, 2025 at 3:47 AM Paolo Abeni <pabeni@...hat.com> wrote:
>
> 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().
>

I meant to say "the skb_page_unref call from __skb_pull_tail is not
reachable for unreadable skbs". This is because __skb_pull_tail early
returns on unreadable skbs.

-- 
Thanks,
Mina