netdev - Re: [PATCH next v1] ipvlan: don't use IDR for generating dev

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Message-Id: <20170106.211836.1613373165796630636.davem@davemloft.net>
Date:   Fri, 06 Jan 2017 21:18:36 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     mahesh@...dewar.net
Cc:     netdev@...r.kernel.org, maheshb@...gle.com, edumazet@...gle.com
Subject: Re: [PATCH next v1] ipvlan: don't use IDR for generating dev_id

From: Mahesh Bandewar <mahesh@...dewar.net>
Date: Fri,  6 Jan 2017 16:33:11 -0800

> From: Mahesh Bandewar <maheshb@...gle.com>
> 
> The patch 009146d117b ("ipvlan: assign unique dev-id for each slave
> device.") used ida_simple_get() to generate dev_ids assigned to the
> slave devices. However (Eric has pointed out that) there is a shortcoming
> with that approach as it always uses the first available ID. This
> becomes a problem when a slave gets deleted and a new slave gets added.
> The ID gets reassigned causing the new slave to get the same link-local
> address. This side-effect is undesirable.
> 
> This patch replaces IDR logic with a simple per-port variable that keeps
> incrementing and wraps around when the MAX (0xFFFE) is reached. The
> only downside is that this is an inefficient (n^2) search if there are
> 64k (or close to 64k) slaves in the system, the dev-id search takes time.
> However having these many devices in the system has it's own challenges.
> 
> Fixes: 009146d117b ("ipvlan: assign unique dev-id for each slave device.")
> Signed-off-by: Mahesh Bandewar <maheshb@...gle.com>

I kind of cringe when I see yet another implementation of an integer ID
allocator.

I think it's much simpler to keep using ida_simple_alloc(), but alongside
it have start point you maintain based upon previous allocations.  Put it
in the ipvl_port, just like dev_id_base, but call it "dev_id_start".

Then your ID allocation sequence becomes:

	err = ida_simple_get(&port->ida, port->dev_id_start, 0xFFFE, GFP_KERNEL);
	if (err < 0)
		err = ida_simple_get(&port->ida, 0, port->dev_id_start, GFP_KERNEL);
	if (err < 0)
		goto destroy_ipvlan_port;

	dev->dev_id = err;

	port->dev_id_start = err;
	if (port->dev_id_start = 0FFFE)
		port->dev_id_start = 0;

Something like that.

Alternatively, IDR/IDA can be extended to have this kind of functionality
too.