linux-kernel - Re: [PATCH net-next v5 6/7] tap: Keep hdr_len in tap_get

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Message-ID: <67a4dece7b7e4_20644429480@willemb.c.googlers.com.notmuch>
Date: Thu, 06 Feb 2025 11:09:50 -0500
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Akihiko Odaki <akihiko.odaki@...nix.com>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
 Jonathan Corbet <corbet@....net>, 
 Jason Wang <jasowang@...hat.com>, 
 "David S. Miller" <davem@...emloft.net>, 
 Eric Dumazet <edumazet@...gle.com>, 
 Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>, 
 "Michael S. Tsirkin" <mst@...hat.com>, 
 Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, 
 Shuah Khan <shuah@...nel.org>, 
 linux-doc@...r.kernel.org, 
 linux-kernel@...r.kernel.org, 
 netdev@...r.kernel.org, 
 kvm@...r.kernel.org, 
 virtualization@...ts.linux-foundation.org, 
 linux-kselftest@...r.kernel.org, 
 Yuri Benditovich <yuri.benditovich@...nix.com>, 
 Andrew Melnychenko <andrew@...nix.com>, 
 Stephen Hemminger <stephen@...workplumber.org>, 
 gur.stavi@...wei.com, 
 devel@...nix.com
Subject: Re: [PATCH net-next v5 6/7] tap: Keep hdr_len in tap_get_user()

Akihiko Odaki wrote:
> On 2025/02/06 6:21, Willem de Bruijn wrote:
> > Akihiko Odaki wrote:
> >> hdr_len is repeatedly used so keep it in a local variable.
> >>
> >> Signed-off-by: Akihiko Odaki <akihiko.odaki@...nix.com>
> > 
> >> @@ -682,11 +683,8 @@ static ssize_t tap_get_user(struct tap_queue *q, void *msg_control,
> >>   	if (msg_control && sock_flag(&q->sk, SOCK_ZEROCOPY)) {
> >>   		struct iov_iter i;
> >>   
> >> -		copylen = vnet_hdr.hdr_len ?
> >> -			tap16_to_cpu(q, vnet_hdr.hdr_len) : GOODCOPY_LEN;
> >> -		if (copylen > good_linear)
> >> -			copylen = good_linear;
> >> -		else if (copylen < ETH_HLEN)
> >> +		copylen = min(hdr_len ? hdr_len : GOODCOPY_LEN, good_linear);
> >> +		if (copylen < ETH_HLEN)
> >>   			copylen = ETH_HLEN;
> > 
> > I forgot earlier: this can also use single line statement
> > 
> >      copylen = max(copylen, ETH_HLEN);
> > 
> > And perhaps easiest to follow is
> > 
> >      copylen = hdr_len ?: GOODCOPY_LEN;
> >      copylen = min(copylen, good_linear);
> >      copylen = max(copylen, ETH_HLEN);
> 
> I introduced the min() usage as it now neatly fits in a line, but I 
> found even clamp() fits so I'll use it in the next version:
> copylen = clamp(hdr_len ?: GOODCOPY_LEN, ETH_HLEN, good_linear);
> 
> Please tell me if you prefer hdr_len ?: GOODCOPY_LEN in a separate line:
> copylen = hdr_len ?: GOODCOPY_LEN;
> copylen = clamp(copylen, ETH_HLEN, good_linear);

Oh nice. I had forgotten about clamp. Even better.