netdev - Re: [PATCH] net: ipv6: avoid errors due to per-cpu atomic alloc

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Date:   Tue, 22 Nov 2016 13:18:52 +0100
From:   Hannes Frederic Sowa <hannes@...essinduktion.org>
To:     Mike Manning <mmanning@...cade.com>, netdev@...r.kernel.org
Subject: Re: [PATCH] net: ipv6: avoid errors due to per-cpu atomic alloc

On 22.11.2016 11:34, Mike Manning wrote:
> Bursts of failures may occur when adding IPv6 routes via Netlink to the
> kernel when testing under scale (e.g. 500 routes lost out of 1M). The
> reason is that percpu.c:pcpu_balance_workfn() is not guaranteed to have
> extended the area map in time for the atomic allocation using percpu.c:
> pcpu_alloc() to succeed. This results in route additions failing with
> an -ENOMEM error.
> 
> While the sender of the Netlink msg to add this route could check for
> an ACK and retransmit in the case of an -ENOMEM error, the latter
> should not occur in the first place if there is plenty of memory. The
> solution is to use non-atomic alloc for rt6_info instead. While the
> client may now be blocked for longer depending on the state of the
> chunk being added to, this work has to be incurred at some point.
> 
> The alternative solution would be to provide configurable parameters
> e.g. via sysctl in percpu.c for default map size, low/high empty pages
> and map margins. For this solution, the map margin sizes need to be
> stored per chunk, as large margins cannot be used if the dynamic early
> slots map size is in use. This is not a preferred solution though, as
> it requires tuning of these parameters to provide sufficient margins to
> avoid -ENOMEM errors depending on system requirements.
> 
> Signed-off-by: Mike Manning <mmanning@...cade.com>
> ---
>  net/ipv6/route.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> index 1b57e11..0e9bb76 100644
> --- a/net/ipv6/route.c
> +++ b/net/ipv6/route.c
> @@ -347,7 +347,7 @@ struct rt6_info *ip6_dst_alloc(struct net *net,
>  	struct rt6_info *rt = __ip6_dst_alloc(net, dev, flags);
>  
>  	if (rt) {
> -		rt->rt6i_pcpu = alloc_percpu_gfp(struct rt6_info *, GFP_ATOMIC);
> +		rt->rt6i_pcpu = alloc_percpu_gfp(struct rt6_info *, GFP_KERNEL);
>  		if (rt->rt6i_pcpu) {
>  			int cpu;

Nak, this doesn't work, as ip6_dst_alloc must be callable from
non-blocking code paths unfortunately.