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Date: Thu, 28 Mar 2024 16:02:12 +0100
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Breno Leitao <leitao@...ian.org>, "David S. Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, "Paolo
Abeni" <pabeni@...hat.com>
CC: <quic_jjohnson@...cinc.com>, <kvalo@...nel.org>, <leon@...nel.org>,
<dennis.dalessandro@...nelisnetworks.com>, Jiri Pirko <jiri@...nulli.us>,
Simon Horman <horms@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
Lorenzo Bianconi <lorenzo@...nel.org>, Coco Li <lixiaoyan@...gle.com>, "open
list:NETWORKING DRIVERS" <netdev@...r.kernel.org>, open list
<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next] net: create a dummy net_device allocator
From: Breno Leitao <leitao@...ian.org>
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@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()
>
> Suggested-by: Jakub Kicinski <kuba@...nel.org>
> Signed-off-by: Breno Leitao <leitao@...ian.org>
> ---
> 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
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