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Date: Fri, 1 Mar 2024 13:58:39 +0100
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Subject: Re: [PATCH] netdev: Use flexible array for trailing private bytes
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To: Eric Dumazet <edumazet@google.com>, Jakub Kicinski <kuba@kernel.org>, Kees
 Cook <keescook@chromium.org>
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From: Eric Dumazet <edumazet@google.com>
Date: Fri, 1 Mar 2024 09:03:55 +0100

> On Fri, Mar 1, 2024 at 7:59 AM Jakub Kicinski <kuba@kernel.org> wrote:
>>
>> On Thu, 29 Feb 2024 13:30:22 -0800 Kees Cook wrote:

Re WARN_ONCE() in netdev_priv(): netdev_priv() is VERY hot, I'm not sure
we want to add checks there. Maybe under CONFIG_DEBUG_NET?

> 
>>> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
>>> index 118c40258d07..b476809d0bae 100644
>>> --- a/include/linux/netdevice.h
>>> +++ b/include/linux/netdevice.h
>>> @@ -1815,6 +1815,8 @@ enum netdev_stat_type {
>>>       NETDEV_PCPU_STAT_DSTATS, /* struct pcpu_dstats */
>>>  };
>>>
>>> +#define      NETDEV_ALIGN            32
>>
>> Unless someone knows what this is for it should go.
>> Align priv to cacheline size.
> 
> +2
> 

Maybe

> #define NETDEV_ALIGN    L1_CACHE_BYTES

#define NETDEV_ALIGN	max(SMP_CACHE_BYTES, 32)

?

(or even max(1 << INTERNODE_CACHE_SHIFT, 32))

> 
> or a general replacement of NETDEV_ALIGN....
> 
> 

+ I'd align both struct net_device AND its private space to
%NETDEV_ALIGN and remove this weird PTR_ALIGN. {k,v}malloc ensures
natural alignment of allocations for at least a couple years already
(IOW if struct net_device is aligned to 64, {k,v}malloc will *always*
return a 64-byte aligned address).

Thanks,
Olek