On 2023/10/09 3:39, Willem de Bruijn wrote:

On Sun, Oct 8, 2023 at 7:22 AM Akihiko Odaki akihiko.odaki@daynix.com wrote:

...

tun_vnet_hash can use this flag to indicate it stored virtio-net hash cache to cb.

Signed-off-by: Akihiko Odaki akihiko.odaki@daynix.com

include/linux/skbuff.h | 2 ++ 1 file changed, 2 insertions(+)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h index 4174c4b82d13..e638f157c13c 100644 --- a/include/linux/skbuff.h +++ b/include/linux/skbuff.h @@ -837,6 +837,7 @@ typedef unsigned char *sk_buff_data_t;

• @truesize: Buffer size
  

• @users: User count - see {datagram,tcp}.c
  

• @extensions: allocated extensions, valid if active_extensions is nonzero
  

• • @tun_vnet_hash: tun stored virtio-net hash cache to cb
    

  */

  struct sk_buff {

@@ -989,6 +990,7 @@ struct sk_buff { #if IS_ENABLED(CONFIG_IP_SCTP) __u8 csum_not_inet:1; #endif

•   __u8                    tun_vnet_hash:1;
  

sk_buff space is very limited.

No need to extend it, especially for code that stays within a single subsystem (tun).

To a lesser extent the same point applies to the qdisc_skb_cb.

I had to extend sk_buff because it does not stay in tun but moves back and forth between qdisc and tun.

The new members of sk_buff and qdisc_skb_cb are stored by tun's ndo_select_queue(). The control will go back to qdisc after ndo_select_queue() function finishes. Eventually tun's ndo_start_xmit() will be called by qdisc and consumes the stored members. qdisc is required to keep the stored members intact.

tun_vnet_hash is a bit special. It is put into sk_buff because ndo_select_queue() is not always called and it may be left uninitialized. ndo_start_xmit() may read some garbage from cb's old user if it is put into cb.