Decrease memory usage with mlockall(MCL_ONFAULT)

[hakavlad]

MCL_ONFAULT (since Linux 4.4)
    Used together with MCL_CURRENT, MCL_FUTURE, or both. Mark all current (with MCL_CURRENT) or future (with MCL_FUTURE) mappings to lock pages when they are faulted in. When used with MCL_CURRENT, all present pages are locked, but mlockall() will not fault in non-present pages. When used with MCL_FUTURE, all future mappings will be marked to lock pages when they are faulted in, but they will not be populated by the lock when the mapping is created. MCL_ONFAULT must be used with either MCL_CURRENT or MCL_FUTURE or both. 

Earlyoom will use 0.7 MiB RSS instead of 2.0 MiB if you use mlockall(MCL_CURRENT | MCL_FUTURE | MCL_ONFAULT).

See also golang/go#28114

Status with MCL_ONFAULT

Name:	earlyoom
...
VmPeak:	    4240 kB
VmSize:	    4232 kB
VmLck:	    4216 kB
VmPin:	       0 kB
VmHWM:	     684 kB
VmRSS:	     684 kB
RssAnon:	      68 kB
RssFile:	     592 kB
RssShmem:	      24 kB

Code:

    if (mlockall(MCL_CURRENT | MCL_FUTURE | MCL_ONFAULT) != 0)
        if (mlockall(MCL_CURRENT | MCL_FUTURE) != 0) // for old kernels
            perror("Could not lock memory - continuing anyway");

[rfjakob]

Very nice. Added in commit 319e959, thanks!