Subject: Re: [PATCH net-next v2] netdevice: define and allocate &net_device _properly_

From: Simon Horman <[email-masked]>
Date: Tue, 9 Jul 2024 19:11:28 +0100

On Tue, Jul 09, 2024 at 05:54:25AM -0700, Breno Leitao wrote:
> From: Alexander Lobakin <[email-masked]>
> 
> In fact, this structure contains a flexible array at the end, but
> historically its size, alignment etc., is calculated manually.
> There are several instances of the structure embedded into other
> structures, but also there's ongoing effort to remove them and we
> could in the meantime declare &net_device properly.
> Declare the array explicitly, use struct_size() and store the array
> size inside the structure, so that __counted_by() can be applied.
> Don't use PTR_ALIGN(), as SLUB itself tries its best to ensure the
> allocated buffer is aligned to what the user expects.
> Also, change its alignment from %NETDEV_ALIGN to the cacheline size
> as per several suggestions on the netdev ML.
> 
> bloat-o-meter for vmlinux:
> 
> free_netdev                                  445     440      -5
> netdev_freemem                                24       -     -24
> alloc_netdev_mqs                            1481    1450     -31
> 
> On x86_64 with several NICs of different vendors, I was never able to
> get a &net_device pointer not aligned to the cacheline size after the
> change.
> 
> Signed-off-by: Alexander Lobakin <[email-masked]>
> Signed-off-by: Breno Leitao <[email-masked]>
> Reviewed-by: Przemek Kitszel <[email-masked]>

Hi Breno,

Some kernel doc warnings from my side.

Flagged by: kernel-doc -none

> ---
> Changelog:
> 
> v2:
>  * Rebased Alexander's patch on top of f750dfe825b90 ("ethtool: provide
>    customized dim profile management").
>  * Removed the ALIGN() of SMP_CACHE_BYTES for sizeof_priv.
> 
> v1:
>  * https://lore.kernel.org/netdev/[email-masked]
> 
>  include/linux/netdevice.h | 12 +++++++-----
>  net/core/dev.c            | 30 ++++++------------------------
>  net/core/net-sysfs.c      |  2 +-
>  3 files changed, 14 insertions(+), 30 deletions(-)
> 
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 93558645c6d0..f0dd499244d4 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -2199,10 +2199,10 @@ struct net_device {
>  	unsigned short		neigh_priv_len;
>  	unsigned short          dev_id;
>  	unsigned short          dev_port;
> -	unsigned short		padded;

padded should also be removed from the Kernel doc for this structure.

> +	int			irq;
> +	u32			priv_len;

And irq and priv_len should be added to the Kernel doc for this structure.

>  
>  	spinlock_t		addr_list_lock;
> -	int			irq;
>  
>  	struct netdev_hw_addr_list	uc;
>  	struct netdev_hw_addr_list	mc;

...