netdev - Re: ndisc_cache garbage collection issue

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Date:   Thu, 2 May 2019 18:59:49 +0100
From:   Tom Hughes <tom@...pton.nu>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        David Ahern <dsahern@...il.com>
Cc:     netdev@...r.kernel.org
Subject: Re: ndisc_cache garbage collection issue

On 02/05/2019 14:19, Eric Dumazet wrote:
> 
> 
> On 5/2/19 5:42 AM, Tom Hughes wrote:
>> I recently upgraded a machine from a 4.20.13 kernel to 5.0.9 and am
>> finding that after a few days I start getting a lot of these messages:
>>
>>    neighbour: ndisc_cache: neighbor table overflow!
>>
>> and IPv6 networking starts to fail intermittently as a result.
>>
>> The neighbour table doesn't appear to have much in it however so I've
>> been looking at the code, and especially your recent changes to garbage
>> collection in the neighbour tables and my working theory is that the
>> value of gc_entries is somehow out of sync with the actual list of what
>> needs to be garbage collected.
>>
>> Looking at the code I think I see a possible way that this could be
>> happening post 8cc196d6ef8 which moved the addition of new entries to
>> the gc list out of neigh_alloc into ___neigh_create.
>>
>> The problem is that neigh_alloc is doing the increment of gc_entries, so
>> if ___neigh_create winds up taking an error path gc_entries will have
>> been incremented but the neighbour will never be added to the gc list.
>>
>> I don't know for sure yet that this is the cause of my problem, but it
>> seems to be incorrect in any case unless I have misunderstood something?
> 
> This seems to match your report : https://patchwork.ozlabs.org/patch/1093973/

That does indeed look like the same thing... I've built a kernel with
that applied now so we'll see how that goes.

Tom

-- 
Tom Hughes (tom@...pton.nu)
http://compton.nu/