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

From: Jonathan Corbet <[email-masked]>
Date: Wed, 8 Jul 2015 15:17:50 -0600

On Wed, 8 Jul 2015 16:34:56 -0400
Eric B Munson <[email-masked]> wrote:

> > 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.

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.

Thanks,

jon