On Tue, 28 Jul 2015, Michal Hocko wrote:

[I am sorry but I didn't get to this sooner.]

On Mon 27-07-15 10:54:09, Eric B Munson wrote:

quoted

Now that VM_LOCKONFAULT is a modifier to VM_LOCKED and
cannot be specified independentally, it might make more sense to mirror
that relationship to userspace.  Which would lead to soemthing like the
following:

A modifier makes more sense.
 

quoted

To lock and populate a region:
mlock2(start, len, 0);

To lock on fault a region:
mlock2(start, len, MLOCK_ONFAULT);

If LOCKONFAULT is seen as a modifier to mlock, then having the flags
argument as 0 mean do mlock classic makes more sense to me.

To mlock current on fault only:
mlockall(MCL_CURRENT | MCL_ONFAULT);

To mlock future on fault only:
mlockall(MCL_FUTURE | MCL_ONFAULT);

To lock everything on fault:
mlockall(MCL_CURRENT | MCL_FUTURE | MCL_ONFAULT);

Makes sense to me. The only remaining and still tricky part would be
the munlock{all}(flags) behavior. What should munlock(MLOCK_ONFAULT)
do? Keep locked and poppulate the range or simply ignore the flag an
just unlock?

I can see some sense to allow munlockall(MCL_FUTURE[|MLOCK_ONFAULT]),
munlockall(MCL_CURRENT) resp. munlockall(MCL_CURRENT|MCL_FUTURE) but
other combinations sound weird to me.

Anyway munlock with flags opens new doors of trickiness.

In the current revision there are no new munlock[all] system calls
introduced.  munlockall() unconditionally cleared both MCL_CURRENT and
MCL_FUTURE before the set and now unconditionally clears all three.
munlock() does the same for VM_LOCK and VM_LOCKONFAULT.  If the user
wants to adjust mlockall flags today, they need to call mlockall a
second time with the new flags, this remains true for mlockall after
this set and the same behavior is mirrored in mlock2.  The only
remaining question I have is should we have 2 new mlockall flags so that
the caller can explicitly set VM_LOCKONFAULT in the mm->def_flags vs
locking all current VMAs on fault.  I ask because if the user wants to
lock all current VMAs the old way, but all future VMAs on fault they
have to call mlockall() twice:

	mlockall(MCL_CURRENT);
	mlockall(MCL_CURRENT | MCL_FUTURE | MCL_ONFAULT);

This has the side effect of converting all the current VMAs to
VM_LOCKONFAULT, but because they were all made present and locked in the
first call, this should not matter in most cases.  The catch is that,
like mmap(MAP_LOCKED), mlockall() does not communicate if mm_populate()
fails.  This has been true of mlockall() from the beginning so I don't
know if it needs more than an entry in the man page to clarify (which I
will add when I add documentation for MCL_ONFAULT).  In a much less
likely corner case, it is not possible in the current setup to request
all current VMAs be VM_LOCKONFAULT and all future be VM_LOCKED.