It is tunable, almost exactly as you suggest. Quoting Documentation/vm/overcommit-accounting:
[S]trict overcommit. The total address space commit for the system is not permitted to exceed swap + a configurable percentage (default is 50) of physical RAM. Depending on the percentage you use, in most situations this means a process will not be killed while accessing pages but will receive errors on memory allocation as appropriate. The overcommit policy is set via the sysctl `vm.overcommit_memory'. The overcommit percentage is set via `vm.overcommit_ratio'.
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