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

[Eric Dumazet]

On Tue, May 7, 2024 at 2:40 PM Alexander Lobakin
<aleksander.lobakin@xxxxxxxxx> wrote:
>
> 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>
> ---

..

> -       p = kvzalloc(alloc_size, GFP_KERNEL_ACCOUNT | __GFP_RETRY_MAYFAIL);
> -       if (!p)
> +       sizeof_priv = ALIGN(sizeof_priv, SMP_CACHE_BYTES);

If we have a __counted_by(priv_len), why do you ALIGN(sizeof_priv,
SMP_CACHE_BYTES) ?

If a driver pretends its private part is 4 bytes, we should get a
warning if 20 bytes are used instead.

You added two ____cacheline_aligned already in net_device already.