netdev - Re: [PATCH v3 net-next 03/14] net: bridge: mst: Support setting and reporting MST port states

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Date:   Mon, 14 Mar 2022 16:58:54 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Tobias Waldekranz <tobias@...dekranz.com>
Cc:     davem@...emloft.net, kuba@...nel.org, Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Ivan Vecera <ivecera@...hat.com>,
        Roopa Prabhu <roopa@...dia.com>,
        Nikolay Aleksandrov <razor@...ckwall.org>,
        Russell King <linux@...linux.org.uk>,
        Ido Schimmel <idosch@...dia.com>,
        Petr Machata <petrm@...dia.com>,
        Cooper Lees <me@...perlees.com>,
        Matt Johnston <matt@...econstruct.com.au>,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        bridge@...ts.linux-foundation.org
Subject: Re: [PATCH v3 net-next 03/14] net: bridge: mst: Support setting and
 reporting MST port states

On Mon, Mar 14, 2022 at 10:52:20AM +0100, Tobias Waldekranz wrote:
> +int br_mst_fill_info(struct sk_buff *skb, struct net_bridge_vlan_group *vg)
> +{
> +	struct net_bridge_vlan *v;
> +	struct nlattr *nest;
> +	unsigned long *seen;
> +	int err = 0;
> +
> +	seen = bitmap_zalloc(VLAN_N_VID, 0);

I see there is precedent in the bridge driver for using dynamic
allocation as opposed to on-stack declaration using DECLARE_BITMAP().
I imagine this isn't just to be "heapsters", but why?

I don't have a very good sense of how much on-stack memory is too much
(a lot probably depends on the expected depth of the call stack too, and here it
doesn't appear to be too deep), but I see that mlxsw_sp_bridge_vxlan_vlan_is_valid()
has a DECLARE_BITMAP(vlans, VLAN_N_VID) too.

The comment applies for callers of br_mst_get_info() too.

> +	if (!seen)
> +		return -ENOMEM;
> +
> +	list_for_each_entry(v, &vg->vlan_list, vlist) {
> +		if (test_bit(v->brvlan->msti, seen))
> +			continue;
> +
> +		nest = nla_nest_start_noflag(skb, IFLA_BRIDGE_MST_ENTRY);
> +		if (!nest ||
> +		    nla_put_u16(skb, IFLA_BRIDGE_MST_ENTRY_MSTI, v->brvlan->msti) ||
> +		    nla_put_u8(skb, IFLA_BRIDGE_MST_ENTRY_STATE, v->state)) {
> +			err = -EMSGSIZE;
> +			break;
> +		}
> +		nla_nest_end(skb, nest);
> +
> +		set_bit(v->brvlan->msti, seen);
> +	}
> +
> +	kfree(seen);
> +	return err;
> +}