linux-kernel - Re: [PATCH net-next v3] netdevice: define and allocate &net_device _properly

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Message-ID: <0664910d-026d-49b8-8b70-a5c881888761@intel.com>
Date: Wed, 10 Jul 2024 16:01:35 +0200
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Breno Leitao <leitao@...ian.org>
CC: "David S. Miller" <davem@...emloft.net>, Eric Dumazet
	<edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
	<pabeni@...hat.com>, Kees Cook <kees@...nel.org>, "Gustavo A. R. Silva"
	<gustavoars@...nel.org>, <keescook@...omium.org>, <horms@...nel.org>,
	<linux-hardening@...r.kernel.org>, Przemek Kitszel
	<przemyslaw.kitszel@...el.com>, Jiri Pirko <jiri@...nulli.us>, "Sebastian
 Andrzej Siewior" <bigeasy@...utronix.de>, Daniel Borkmann
	<daniel@...earbox.net>, Lorenzo Bianconi <lorenzo@...nel.org>, Johannes Berg
	<johannes.berg@...el.com>, "open list:NETWORKING [GENERAL]"
	<netdev@...r.kernel.org>, open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next v3] netdevice: define and allocate &net_device
 _properly_

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

> From: Alexander Lobakin <aleksander.lobakin@...el.com>
> 
> 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@...el.com>
> Signed-off-by: Breno Leitao <leitao@...ian.org>
> Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@...el.com>
> Reviewed-by: Eric Dumazet <edumazet@...gle.com>
> Reviewed-by: Kees Cook <kees@...nel.org>

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