Re: [PATCH net-next] net: create a dummy net_device allocator

[Alexander Lobakin]

From: Breno Leitao <leitao@xxxxxxxxxx>
Date: Wed, 27 Mar 2024 13:08:04 -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@xxxxxxxxxx/
> 
> 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()
> 
> Suggested-by: Jakub Kicinski <kuba@xxxxxxxxxx>
> Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
> ---
>  include/linux/netdevice.h |  3 +++
>  net/core/dev.c            | 55 ++++++++++++++++++++++++++-------------
>  2 files changed, 40 insertions(+), 18 deletions(-)
> 
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index c6f6ac779b34..f4226a99a146 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -4545,6 +4545,9 @@ static inline void netif_addr_unlock_bh(struct net_device *dev)
>  
>  void ether_setup(struct net_device *dev);
>  
> +/* Allocate dummy net_device */
> +struct net_device *alloc_netdev_dummy(int sizeof_priv, const char *name);
> +
>  /* Support for loadable net-drivers */
>  struct net_device *alloc_netdev_mqs(int sizeof_priv, const char *name,
>  				    unsigned char name_assign_type,
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 0766a245816b..df2484bbc041 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -10340,25 +10340,12 @@ int register_netdevice(struct net_device *dev)
>  }
>  EXPORT_SYMBOL(register_netdevice);
>  
> -/**
> - *	init_dummy_netdev	- init a dummy network device for NAPI
> - *	@dev: device to init
> - *
> - *	This takes a network device structure and initialize the minimum
> - *	amount of fields so it can be used to schedule NAPI polls without
> - *	registering a full blown interface. This is to be used by drivers
> - *	that need to tie several hardware interfaces to a single NAPI
> - *	poll scheduler due to HW limitations.
> +/* Initialize the core of a dummy net device.
> + * This is useful if you are calling this function after alloc_netdev(),
> + * since it does not memset the net_device fields.
>   */
> -void init_dummy_netdev(struct net_device *dev)
> +static void init_dummy_netdev_core(struct net_device *dev)
>  {
> -	/* Clear everything. Note we don't initialize spinlocks
> -	 * are they aren't supposed to be taken by any of the
> -	 * NAPI code and this dummy netdev is supposed to be
> -	 * only ever used for NAPI polls
> -	 */
> -	memset(dev, 0, sizeof(struct net_device));
> -
>  	/* make sure we BUG if trying to hit standard
>  	 * register/unregister code path
>  	 */
> @@ -10379,8 +10366,28 @@ void init_dummy_netdev(struct net_device *dev)
>  	 * its refcount.
>  	 */
>  }
> -EXPORT_SYMBOL_GPL(init_dummy_netdev);
>  
> +/**
> + *	init_dummy_netdev	- init a dummy network device for NAPI
> + *	@dev: device to init
> + *
> + *	This takes a network device structure and initialize the minimum
> + *	amount of fields so it can be used to schedule NAPI polls without
> + *	registering a full blown interface. This is to be used by drivers
> + *	that need to tie several hardware interfaces to a single NAPI
> + *	poll scheduler due to HW limitations.
> + */
> +void init_dummy_netdev(struct net_device *dev)
> +{
> +	/* Clear everything. Note we don't initialize spinlocks
> +	 * are they aren't supposed to be taken by any of the
> +	 * NAPI code and this dummy netdev is supposed to be
> +	 * only ever used for NAPI polls
> +	 */
> +	memset(dev, 0, sizeof(struct net_device));
> +	init_dummy_netdev_core(dev);
> +}
> +EXPORT_SYMBOL_GPL(init_dummy_netdev);
>  
>  /**
>   *	register_netdev	- register a network device
> @@ -10991,6 +10998,18 @@ 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
> + * @name: device name format string
> + */
> +struct net_device *alloc_netdev_dummy(int sizeof_priv, const char *name)

Since the users of init_dummy_netdev embed &net_device into their
private structures, do we need sizeof_priv here at all? Or maybe we
could unconditionally pass 0?

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

As Jakub mentioned, you need to introduce consumers of the functionality
you add within the same series. Personally, I'd like to see a series
with agressive conversion of all the affected drivers from
init_dummy_netdev() to alloc_dummy_netdev() and final removal of
init_dummy_netdev() :D

(and then a followup which converts &net_device to proper flex arrays)

Thanks,
Olek