Manual Pages  — MLOCK

After an mlock() system call, the indicated pages will cause neither a non-resident page nor address-translation fault until they are unlocked. They may still cause protection-violation faults or TLB-miss faults on architectures with software-managed TLBs. The physical pages remain in memory until all locked mappings for the pages are removed. Multiple processes may have the same physical pages locked via their own virtual address mappings. A single process may likewise have pages multiply-locked via different virtual mappings of the same physical pages. Unlocking is performed explicitly by munlock() or implicitly by a call to munmap() which deallocates the unmapped address range. Locked mappings are not inherited by the child process after a fork(2).

Since physical memory is a potentially scarce resource, processes are limited in how much they can lock down. The amount of memory that a single process can mlock() is limited by both the per-process RLIMIT_MEMLOCK resource limit and the system-wide "wired pages" limit vm.max_user_wired. vm.max_user_wired applies to the system as a whole, so the amount available to a single process at any given time is the difference between vm.max_user_wired and vm.stats.vm.v_user_wire_count.

If security.bsd.unprivileged_mlock is set to 0 these calls are only available to the super-user.