netdev - Re: [Question] neighbor entry doesn't switch to the STALE state after the reachable timer expires

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Date:   Thu, 2 Feb 2023 16:27:30 +0800
From:   Zhang Changzhong <zhangchangzhong@...wei.com>
To:     Julian Anastasov <ja@....bg>
CC:     Network Development <netdev@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        "Denis V. Lunev" <den@...nvz.org>,
        Nikolay Aleksandrov <razor@...ckwall.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        YueHaibing <yuehaibing@...wei.com>,
        Zhang Changzhong <zhangchangzhong@...wei.com>
Subject: Re: [Question] neighbor entry doesn't switch to the STALE state after
 the reachable timer expires

On 2023/2/1 0:13, Julian Anastasov wrote:
> 
>> Just curious, why did you choose 'jiffies - MAX_JIFFY_OFFSET + 86400 * HZ'
>> as the value of 'mint'?
> 
> 	It is too arbitrary :) Probably, just 'jiffies - MAX_JIFFY_OFFSET'
> is enough or something depending on HZ/USER_HZ. I added 1 day for
> timer to advance without leaving confirmed time behind the
> jiffies - MAX_JIFFY_OFFSET zone but it is not needed.
> 
> 	What limits play here:
> 
> - the HZ/USER_HZ difference: jiffies_to_clock_t reports the 3 times
> to user space, so we want to display values as large as possible.
> Any HZ > 100 for USER_HZ=100 works for the jiffies - MAX_JIFFY_OFFSET.
> HZ=100 does not work.
> 
> - users can use large values for sysctl vars which can keep the timer
> running for long time and reach some outdated confirmed time
> before neigh_add_timer() is called to correct it
> 
> 	If we choose mint = jiffies - MAX_JIFFY_OFFSET,
> for 32-bit we will have:
> 
> Past                     Future
> ++++++++++++++++++++++++++++++++++++++++++++++++++++
> |  49  days   |  49 days  |         99 days        |
> ++++++++++++++^+++++++++++^+++++++++++++++++++++++++
>               ^           ^
> DELAY+PROBE   |           |
>             mint         now
> 
> - used/confirmed times should be up to 49 days behind jiffies but
> we have 49 days to stay in timer without correcting them,
> so they can go up to 99 days in the past before going in
> the future and trigger the problem
> 
> - as we avoid the checks in neigh_timer_handler to save CPU cycles,
> one needs crazy sysctl settings to keep the timer in DELAY+PROBE
> states for 49 days. With default settings, it is no more than
> half minute. In this case even
> mint = jiffies - LONG_MAX + 86400 * HZ should work.
> 
> - REACHABLE state extends while confirmed time advances,
> otherwise PROBE will need ARP reply to recheck the
> times in neigh_add_timer while entering REACHABLE again
> 

Wow, thank you so much for the detailed explanation! Are you planning
to mainline it?

Regards,
Changzhong