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

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Message-ID: <20150709184635.GE4669@akamai.com>
Date:	Thu, 9 Jul 2015 14:46:35 -0400
From:	Eric B Munson <emunson@...mai.com>
To:	Jonathan Corbet <corbet@....net>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>,
	Vlastimil Babka <vbabka@...e.cz>, linux-alpha@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mips@...ux-mips.org,
	linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	sparclinux@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
	linux-mm@...ck.org, linux-arch@...r.kernel.org,
	linux-api@...r.kernel.org
Subject: 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 <emunson@...mai.com> 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.

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

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