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

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Date: Wed, 09 Aug 2023 09:18:22 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Tahsin Erdogan <trdgn@...zon.com>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
 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>, 
 Herbert Xu <herbert@...dor.apana.org.au>
Cc: Tahsin Erdogan <trdgn@...zon.com>, 
 netdev@...r.kernel.org, 
 linux-kernel@...r.kernel.org
Subject: RE: [PATCH v3] tun: avoid high-order page allocation for packet
 header

Tahsin Erdogan wrote:
> When GSO is not enabled

Not GSO, but gso.hdr_len, which is a feature of IFF_VNET_HDR.

VIRTIO_NET_HDR_GSO_* does not need to be enabled to use the
header length field.

> and a packet is transmitted via writev(), all
> payload is treated as header which requires a contiguous memory allocation.
> This allocation request is harder to satisfy, and may even fail if there is
> enough fragmentation.
> 
> Note that sendmsg() code path limits the linear copy length, so this change
> makes writev() and sendmsg() more consistent.

This is not specific to writev(), equally to more common write().

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?

> Signed-off-by: Tahsin Erdogan <trdgn@...zon.com>
> ---
> v3: rebase to latest net-next
> v2: replace linear == 0 with !linear
> v1: https://lore.kernel.org/all/20230726030936.1587269-1-trdgn@amazon.com/
>  drivers/net/tun.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 5beb6b5dd7e5..53d19c958a20 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1523,7 +1523,7 @@ static struct sk_buff *tun_alloc_skb(struct tun_file *tfile,
>  	int err;
>  
>  	/* Under a page?  Don't bother with paged skb. */
> -	if (prepad + len < PAGE_SIZE || !linear)
> +	if (prepad + len < PAGE_SIZE)
>  		linear = len;
>  
>  	if (len - linear > MAX_SKB_FRAGS * (PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER))
> @@ -1913,6 +1913,9 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
>  			 */
>  			zerocopy = false;
>  		} else {
> +			if (!linear)
> +				linear = min_t(size_t, good_linear, copylen);
> +
>  			skb = tun_alloc_skb(tfile, align, copylen, linear,
>  					    noblock);
>  		}
> -- 
> 2.41.0
>