One would hope that any machine hosting an in-memory database has gobloads of RAM and not much else to do. :) [Still, this is what replication is for! "In-memory database" is only a scary concept if you don't have any hot slaves.]
One might have 2/3 of memory used, with well known database size growth. Then a runaway process could use 1/3 of memory, cause trouble, but it's still the DB that gets killed.
This isn't disastrous of course (replication + snapshots + redis aof), but still annoying.
Another thing you can do, in this case, is to enable swap system-wide, then put just the database process into a cgroup with memory.swappiness = 0.
Thus, the database itself will never degrade due to spilling to disk, but all other processes might. But if it's a DB box, that won't precisely matter.
