Re: [PATCH net-next v7 2/9] net: add get_netmem/put_netmem support

Mina Almasry
2025-03-13T15:38:13Z

On Thu, Mar 13, 2025 at 3:47 AM Paolo Abeni <pabeni@redhat.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@google.com>
> > Acked-by: Stanislav Fomichev <sdf@fomichev.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