With the current implementation, minijail's generate_seccomp_policy.py generates policy like this:
  mmap: arg2 == PROT_READ || arg2 == PROT_READ|PROT_EXEC || arg2 == PROT_READ|PROT_WRITE || arg2 == PROT_NONE

This is inefficient because it performs 4 additional checks on every mmap syscall.
generate_seccomp_policy.py should be fixed to output more efficient policy.