linux-kernel - Re: [PATCH net-next 3/4] tun: use napi_build_skb in __tun_build

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Message-ID: <681bc77c96437_20dc642942a@willemb.c.googlers.com.notmuch>
Date: Wed, 07 May 2025 16:50:04 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Jon Kohler <jon@...anix.com>, 
 ast@...nel.org, 
 daniel@...earbox.net, 
 davem@...emloft.net, 
 kuba@...nel.org, 
 hawk@...nel.org, 
 john.fastabend@...il.com, 
 netdev@...r.kernel.org, 
 bpf@...r.kernel.org, 
 jon@...anix.com, 
 aleksander.lobakin@...el.com, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
 Jason Wang <jasowang@...hat.com>, 
 Andrew Lunn <andrew+netdev@...n.ch>, 
 Eric Dumazet <edumazet@...gle.com>, 
 Paolo Abeni <pabeni@...hat.com>, 
 linux-kernel@...r.kernel.org (open list), 
 hawk@...nel.org
Subject: Re: [PATCH net-next 3/4] tun: use napi_build_skb in __tun_build_skb

Jon Kohler wrote:
> Use napi_build_skb for small payload SKBs that end up using the
> tun_build_skb path.
> 
> Signed-off-by: Jon Kohler <jon@...anix.com>
> ---
>  drivers/net/tun.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index f7f7490e78dc..7b13d4bf5374 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1538,7 +1538,11 @@ static struct sk_buff *__tun_build_skb(struct tun_file *tfile,
>  				       int buflen, int len, int pad,
>  				       int metasize)
>  {
> -	struct sk_buff *skb = build_skb(buf, buflen);
> +	struct sk_buff *skb;
> +
> +	local_bh_disable();
> +	skb = napi_build_skb(buf, buflen);
> +	local_bh_enable();

The goal of this whole series seems to be to use the percpu skb cache
for bulk alloc.

As all these helpers' prefix indicates, they are meant to be used with
NAPI. Not sure using them on a tun write() datapath is deemed
acceptable. Or even correct. Perhaps the infrastructure authors have
an opinion.

>From commit 795bb1c00dd3 ("net: bulk free infrastructure for NAPI
context, use napi_consume_skb") it does appear that technically all
that is needed is to be called in softirq context.