Thanks to excellent diagnosis by Eduard Guzovsky.
The core problem is that on a network with lots of active
multicast traffic, the neighbour cache can fill up. If
we try to allocate a new route and thus neighbour cache
entry, the bog-standard GC attempt the neighbour layer does
in ineffective because route entries hold a reference
to the existing neighbour entries and GC can only liberate
entries with no references.
IPV4 already has a way to handle this, by doing a route cache
GC in such situations (when neigh attach returns -ENOBUFS).
So simply mimick this on the ipv6 side.
Tested-by: Eduard Guzovsky <eguzovsky@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
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