netdev - Re: [PATCH net-next 01/12] vxlan: Add a function to init switchdev_notifier_vxlan_fdb

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Date:   Wed, 05 Dec 2018 20:12:37 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     idosch@...lanox.com
Cc:     netdev@...r.kernel.org, bridge@...ts.linux-foundation.org,
        jiri@...lanox.com, petrm@...lanox.com, roopa@...ulusnetworks.com,
        nikolay@...ulusnetworks.com, mlxsw@...lanox.com
Subject: Re: [PATCH net-next 01/12] vxlan: Add a function to init
 switchdev_notifier_vxlan_fdb_info

From: Ido Schimmel <idosch@...lanox.com>
Date: Wed, 5 Dec 2018 15:50:23 +0000

> +static struct switchdev_notifier_vxlan_fdb_info
> +vxlan_fdb_switchdev_notifier_info(const struct vxlan_dev *vxlan,
> +				  const struct vxlan_fdb *fdb,
> +				  const struct vxlan_rdst *rd)
> +{
> +	struct switchdev_notifier_vxlan_fdb_info fdb_info = {
> +		.info.dev = vxlan->dev,
> +		.remote_ip = rd->remote_ip,
> +		.remote_port = rd->remote_port,
> +		.remote_vni = rd->remote_vni,
> +		.remote_ifindex = rd->remote_ifindex,
> +		.vni = fdb->vni,
> +		.offloaded = rd->offloaded,
> +		.added_by_user = fdb->flags & NTF_VXLAN_ADDED_BY_USER,
> +	};
> +
> +	memcpy(fdb_info.eth_addr, fdb->eth_addr, ETH_ALEN);
> +	return fdb_info;
> +}

Never return a structure by value from a function.  Do you have any idea
how the compiler implement this?

In the one call site in vxlan.c after this patch is applied, _two_
switchdev_notifier_cxlan_fdb_info objects are allocated on the stack.

vxlan_fdb_swtich_dev_notifier_info() fills in the first one, and
then the caller copies that into the second one.

A huge waste.

Please just pass "&info" as an argument.