Date: Tue, 28 Jul 2015 13:17:25 +0200
From: Michal Hocko <mhocko@kernel.org>
To: Eric B Munson <emunson@akamai.com>
Cc: Vlastimil Babka <vbabka@suse.cz>,
        Andrew Morton <akpm@linux-foundation.org>,
        Shuah Khan <shuahkh@osg.samsung.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Jonathan Corbet <corbet@lwn.net>, Ralf Baechle <ralf@linux-mips.org>,
        linux-alpha@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mips@linux-mips.org, linux-parisc@vger.kernel.org,
        linuxppc-dev@lists.ozlabs.org, sparclinux@vger.kernel.org,
        linux-xtensa@linux-xtensa.org, linux-mm@kvack.org,
        linux-arch@vger.kernel.org, linux-api@vger.kernel.org
Subject: Re: [PATCH V5 0/7] Allow user to request memory to be locked on page
 fault
Message-ID: <20150728111725.GG24972@dhcp22.suse.cz>
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 <20150727133555.GA17133@akamai.com>
 <55B63D37.20303@suse.cz>
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[I am sorry but I didn't get to this sooner.]

On Mon 27-07-15 10:54:09, Eric B Munson wrote:
> 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.
 
> 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.
-- 
Michal Hocko
SUSE Labs
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