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

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Message-ID: <87y21clewm.fsf@waldekranz.com>
Date:   Mon, 14 Mar 2022 16:42:01 +0100
From:   Tobias Waldekranz <tobias@...dekranz.com>
To:     Vladimir Oltean <olteanv@...il.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 16:58, Vladimir Oltean <olteanv@...il.com> wrote:
> 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.

In v4, things become even worse, as I need to allocate the bitmap in a
context where I can't return an error. So if it's ok to keep it on the
stack, then that would be great.

Here's the code in question:

size_t br_mst_info_size(const struct net_bridge_vlan_group *vg)
{
	const struct net_bridge_vlan *v;
	unsigned long *seen;
	size_t sz;

	seen = bitmap_zalloc(VLAN_N_VID, 0);
	if (WARN_ON(!seen))
		return 0;

	/* IFLA_BRIDGE_MST */
	sz = nla_total_size(0);

	list_for_each_entry(v, &vg->vlan_list, vlist) {
		if (test_bit(v->brvlan->msti, seen))
			continue;

		/* IFLA_BRIDGE_MST_ENTRY */
		sz += nla_total_size(0) +
			/* IFLA_BRIDGE_MST_ENTRY_MSTI */
			nla_total_size(sizeof(u16)) +
			/* IFLA_BRIDGE_MST_ENTRY_STATE */
			nla_total_size(sizeof(u8));

		__set_bit(v->brvlan->msti, seen);
	}

	kfree(seen);
	return sz;
}