netdev - Re: [PATCH v3] tun: avoid high-order page allocation for packet header

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Date: Wed, 09 Aug 2023 10:01:24 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: "Erdogan, Tahsin" <trdgn@...zon.com>, 
 "herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>, 
 "jasowang@...hat.com" <jasowang@...hat.com>, 
 "willemdebruijn.kernel@...il.com" <willemdebruijn.kernel@...il.com>, 
 "davem@...emloft.net" <davem@...emloft.net>, 
 "kuba@...nel.org" <kuba@...nel.org>, 
 "edumazet@...gle.com" <edumazet@...gle.com>, 
 "pabeni@...hat.com" <pabeni@...hat.com>
Cc: "netdev@...r.kernel.org" <netdev@...r.kernel.org>, 
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] tun: avoid high-order page allocation for packet
 header

Erdogan, Tahsin wrote:
> On Wed, 2023-08-09 at 09:18 -0400, Willem de Bruijn wrote:
> > Tun sendmsg is a special case, only used by vhost-net from inside the
> > kernel. Arguably consistency with packet_snd/packet_alloc_skb would
> > be
> > more important. That said, this makes sense to me. I assume your
> > configuring a device with very large MTU?
> 
> That's right. I am setting MTU to 9100 in my test.

Makes sense. That's not even that large.

Please address the commit message points about virtio_net_hdr.hdr_len
and write() vs writev().

A writev() specific solution could even take the first iov length as
hint. Note that I'm not suggesting that. IFF_NAPI_FRAGS already does
exactly that, plus the geometry of subsequent frags.