netdev - Re: [PATCH net] inet: bring NLM_DONE out to a separate recv() in inet_dump

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Date: Mon, 3 Jun 2024 16:22:19 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: David Ahern <dsahern@...nel.org>, Stephen Hemminger <stephen@...workplumber.org>, davem@...emloft.net, 
	netdev@...r.kernel.org, pabeni@...hat.com, 
	Jaroslav Pulchart <jaroslav.pulchart@...ddata.com>
Subject: Re: [PATCH net] inet: bring NLM_DONE out to a separate recv() in inet_dump_ifaddr()

On Mon, Jun 3, 2024 at 3:54 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Sun, 2 Jun 2024 15:21:02 -0700 Jakub Kicinski wrote:
> > Netlink is full of legacy behavior, the only way to make it usable
> > in modern environments is to let new families not repeat the mistakes.
> > That's why I'd really rather not add a workaround at the af_netlink
> > level. Why would ethtool (which correctly coalesced NLM_DONE from day 1)
> > suddenly start needed another recv(). A lot of the time the entire dump
> > fits in one skb.
> >
> > If you prefer to sacrifice all of rtnetlink (some of which, to be clear,
> > has also been correctly coded from day 1) - we can add a trampoline for
> > rtnetlink dump handlers?
>
> Hi Eric, how do you feel about this approach? It would also let us
> extract the "RTNL unlocked dump" handling from af_netlink.c, which
> would be nice.

Sure, I have not thought of af_netlink

>
> BTW it will probably need to be paired with fixing the
> for_each_netdev_dump() foot gun, maybe (untested):
>

I confess I am a bit lost : this part relates to your original submission,
when you set "ctx->ifindex = ULONG_MAX;"  in inet_dump_ifaddr() ?


> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -3025,7 +3025,8 @@ int call_netdevice_notifiers_info(unsigned long val,
>  #define net_device_entry(lh)   list_entry(lh, struct net_device, dev_list)
>
>  #define for_each_netdev_dump(net, d, ifindex)                          \
> -       xa_for_each_start(&(net)->dev_by_index, (ifindex), (d), (ifindex))
> +       for (; (d = xa_find(&(net)->dev_by_index, &ifindex,             \
> +                           ULONG_MAX, XA_PRESENT)); ifindex++)
>
>  static inline struct net_device *next_net_device(struct net_device *dev)
>  {
>