Re: [PATCH net-next v3] netdevice: define and allocate &net_device _properly_

[Alexander Lobakin]

From: Breno Leitao <leitao@xxxxxxxxxx>
Date: Wed, 10 Jul 2024 04:30:28 -0700

> From: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx>
> 
> 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 <aleksander.lobakin@xxxxxxxxx>
> Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
> Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@xxxxxxxxx>
> Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>
> Reviewed-by: Kees Cook <kees@xxxxxxxxxx>

You did a great job converting embedded &net_devices, thanks a lot!

I hope SLUB won't return you a non-cacheline-aligned pointer after that
you removed SMP_CACHE_ALIGN(sizeof_priv), right?

Olek