Re: [PATCH V3 3/5] mm: mlock: Introduce VM_LOCKONFAULT and add mlock flags to enable it

On Wed, 08 Jul 2015, Jonathan Corbet wrote:

On Wed, 8 Jul 2015 16:34:56 -0400
Eric B Munson [off-list ref] wrote:

quoted

quoted

Quick, possibly dumb question: I've been beating my head against these for
a little bit, and I can't figure out what's supposed to happen in this
case:

	mlock2(addr, len, MLOCK_ONFAULT);
	munlock2(addr, len, MLOCK_LOCKED);

It looks to me like it will clear VM_LOCKED without actually unlocking any
pages.  Is that the intended result?  

This is not quite right, what happens when you call munlock2(addr, len,
MLOCK_LOCKED); is we call apply_vma_flags(addr, len, VM_LOCKED, false).

From your explanation, it looks like what I said *was* right...what I was
missing was the fact that VM_LOCKED isn't set in the first place.  So that
call would be a no-op, clearing a flag that's already cleared.

Sorry, I misread the original.  You are correct with the addition that
the call to munlock2(MLOCK_LOCKED) is a noop in this case.

One other question...if I call mlock2(MLOCK_ONFAULT) on a range that
already has resident pages, I believe that those pages will not be locked
until they are reclaimed and faulted back in again, right?  I suspect that
could be surprising to users.

That is the case.  I am looking into what it would take to find only the
present pages in a range and lock them, if that is the behavior that is
preferred I can include it in the updated series.

Thanks,

jon