Re: [PATCH] neighbour: guarantee the localhost connections be established successfully even the ARP table is full

[Eric Dumazet <edumazet@google.com>]

On Tue, Apr 16, 2024 at 11:54 AM Zheng Li <lizheng043@gmail.com> wrote:
>
> From: Zheng Li <James.Z.Li@Dell.com>
>
> Inter-process communication on localhost should be established successfully
> even the ARP table is full, many processes on server machine use the
> localhost to communicate such as command-line interface (CLI),
> servers hope all CLI commands can be executed successfully even the arp
> table is full. Right now CLI commands got timeout when the arp table is
> full. Set the parameter of exempt_from_gc to be true for LOOPBACK net
> device to keep localhost neigh in arp table, not removed by gc.
>
> the steps of reproduced:
> server with "gc_thresh3 = 1024" setting, ping server from more than 1024
> same netmask Lan IPv4 addresses, run "ssh localhost" on console interface,
> then the command will get timeout.
>
> Signed-off-by: Zheng Li <James.Z.Li@Dell.com>
> ---
>  net/core/neighbour.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/net/core/neighbour.c b/net/core/neighbour.c
> index 552719c3bbc3..47d07b122f7a 100644
> --- a/net/core/neighbour.c
> +++ b/net/core/neighbour.c
> @@ -734,7 +734,9 @@ ___neigh_create(struct neigh_table *tbl, const void *pkey,
>  struct neighbour *__neigh_create(struct neigh_table *tbl, const void *pkey,
>                                  struct net_device *dev, bool want_ref)
>  {
> -       return ___neigh_create(tbl, pkey, dev, 0, false, want_ref);
> +       bool exempt_from_gc = !!(dev->flags & IFF_LOOPBACK);
> +
> +       return ___neigh_create(tbl, pkey, dev, 0, exempt_from_gc, want_ref);
>  }
>  EXPORT_SYMBOL(__neigh_create);
>

Hmmm...

Loopback IPv4 can hold 2^24 different addresses, that is 16384 * 1024