linux-kernel - Re: [PATCH net-next v3 1/5] net: create a dummy net

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Date: Thu, 4 Apr 2024 18:40:33 +0200
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Breno Leitao <leitao@...ian.org>
CC: <kuba@...nel.org>, <davem@...emloft.net>, <pabeni@...hat.com>,
	<edumazet@...gle.com>, <elder@...nel.org>,
	<linux-arm-kernel@...ts.infradead.org>, <linux-mediatek@...ts.infradead.org>,
	<nbd@....name>, <sean.wang@...iatek.com>, <Mark-MC.Lee@...iatek.com>,
	<lorenzo@...nel.org>, <taras.chornyi@...ision.eu>,
	<quic_jjohnson@...cinc.com>, <kvalo@...nel.org>, <leon@...nel.org>,
	<dennis.dalessandro@...nelisnetworks.com>, <linux-kernel@...r.kernel.org>,
	<netdev@...r.kernel.org>, <bpf@...r.kernel.org>, Jiri Pirko
	<jiri@...nulli.us>, Simon Horman <horms@...nel.org>, Daniel Borkmann
	<daniel@...earbox.net>, Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH net-next v3 1/5] net: create a dummy net_device allocator

From: Breno Leitao <leitao@...ian.org>
Date: Thu, 4 Apr 2024 04:48:41 -0700

> It is impossible to use init_dummy_netdev together with alloc_netdev()
> as the 'setup' argument.
> 
> This is because alloc_netdev() initializes some fields in the net_device
> structure, and later init_dummy_netdev() memzero them all. This causes
> some problems as reported here:
> 
> 	https://lore.kernel.org/all/20240322082336.49f110cc@kernel.org/
> 
> Split the init_dummy_netdev() function in two. Create a new function called
> init_dummy_netdev_core() that does not memzero the net_device structure.
> Then have init_dummy_netdev() memzero-ing and calling
> init_dummy_netdev_core(), keeping the old behaviour.
> 
> init_dummy_netdev_core() is the new function that could be called as an
> argument for alloc_netdev().
> 
> Also, create a helper to allocate and initialize dummy net devices,
> leveraging init_dummy_netdev_core() as the setup argument. This function
> basically simplify the allocation of dummy devices, by allocating and
> initializing it. Freeing the device continue to be done through
> free_netdev()

[...]

> @@ -11063,6 +11070,17 @@ void free_netdev(struct net_device *dev)
>  }
>  EXPORT_SYMBOL(free_netdev);
>  
> +/**
> + * alloc_netdev_dummy - Allocate and initialize a dummy net device.
> + * @sizeof_priv: size of private data to allocate space for
> + */
> +struct net_device *alloc_netdev_dummy(int sizeof_priv)

Repeating my question from the previous thread: I see that in your
series you always pass 0 as @sizeof_priv, does it make sense to have
this argument or we can just pass 0 here to alloc_netdev() unconditionally?
Drivers that have &net_device embedded can't have any private data there
anyway.

> +{
> +	return alloc_netdev(sizeof_priv, "dummy#", NET_NAME_UNKNOWN,
> +			    init_dummy_netdev_core);
> +}
> +EXPORT_SYMBOL_GPL(alloc_netdev_dummy);
> +
>  /**
>   *	synchronize_net -  Synchronize with packet receive processing
>   *

Thanks,
Olek