Re: [PATCH] net: make neigh_priv_len in struct net_device 16bit instead of 8bit

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 12 Dec 2013 10:15:59 +0100
Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote:

> neigh_priv_len is defined as u8. With all debug enabled struct
> ipoib_neigh has 200 bytes. The largest part is sk_buff_head with 96
> bytes and here the spinlock with 72 bytes.
> The size value still fits in this u8 leaving some room for more.
> 
> On -RT struct ipoib_neigh put on weight and has 392 bytes. The main
> reason is sk_buff_head with 288 and the fatty here is spinlock with 192
> bytes. This does no longer fit into into neigh_priv_len and gcc
> complains.
> 
> This patch changes neigh_priv_len from being 8bit to 16bit. Since the
> following element (dev_id) is 16bit followed by a spinlock which is
> aligned, the struct remains with a total size of 3200 (allmodconfig) /
> 2048 (with as much debug off as possible) bytes on x86-64.
> On x86-32 the struct is 1856 (allmodconfig) / 1216 (with as much debug
> off as possible) bytes long. The numbers were gained with and without
> the patch to prove that this change does not increase the size of the
> struct.
> 
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
>  include/linux/netdevice.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 0ca8100..56da573 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -1255,7 +1255,7 @@ struct net_device {
>  	unsigned char		perm_addr[MAX_ADDR_LEN]; /* permanent hw address */

MAX_ADDR_LEN is 32, thus it ends with proper alignment.

>  	unsigned char		addr_assign_type; /* hw address assignment type */
>  	unsigned char		addr_len;	/* hardware address length	*/
> -	unsigned char		neigh_priv_len;
> +	unsigned short		neigh_priv_len;

That means the new change should not affect the structure at all, as
the original structure has three chars followed by a short. This would
produce a 1 byte hole.

I wonder if Arnaldo's pahole tools would show this?

-- Steve

>  	unsigned short          dev_id;		/* Used to differentiate devices
>  						 * that share the same link
>  						 * layer address