Subject: Re: [PATCH V5 5/7] mm: mmap: Add mmap flag to request VM_LOCKONFAULT

From: Eric B Munson <[email-masked]>
Date: Mon, 27 Jul 2015 10:11:31 -0400

On Mon, 27 Jul 2015, Kirill A. Shutemov wrote:

> On Mon, Jul 27, 2015 at 09:41:26AM -0400, Eric B Munson wrote:
> > On Mon, 27 Jul 2015, Kirill A. Shutemov wrote:
> > 
> > > On Fri, Jul 24, 2015 at 05:28:43PM -0400, Eric B Munson wrote:
> > > > The cost of faulting in all memory to be locked can be very high when
> > > > working with large mappings.  If only portions of the mapping will be
> > > > used this can incur a high penalty for locking.
> > > > 
> > > > Now that we have the new VMA flag for the locked but not present state,
> > > > expose it as an mmap option like MAP_LOCKED -> VM_LOCKED.
> > > 
> > > As I mentioned before, I don't think this interface is justified.
> > > 
> > > MAP_LOCKED has known issues[1]. The MAP_LOCKED problem is not necessary
> > > affects MAP_LOCKONFAULT, but still.
> > > 
> > > Let's not add new interface unless it's demonstrably useful.
> > > 
> > > [1] http://lkml.kernel.org/g/[email-masked]
> > 
> > I understand and should have been more explicit.  This patch is still
> > included becuase I have an internal user that wants to see it added.
> > The problem discussed in the thread you point out does not affect
> > MAP_LOCKONFAULT because we do not attempt to populate the region with
> > MAP_LOCKONFAULT.
> > 
> > As I told Vlastimil, if this is a hard NAK with the patch I can work
> > with that.  Otherwise I prefer it stays.
> 
> That's not how it works.

I am not sure what you mean here.  I have a user that will find this
useful and MAP_LOCKONFAULT does not suffer from the problem you point
out.  I do not understand your NAK but thank you for explicit about it.

> 
> Once an ABI added to the kernel it stays there practically forever.
> Therefore it must be useful to justify maintenance cost. I don't see it
> demonstrated.

I understand this, and I get that you do not like MAP_LOCKED, but I do
not see how your dislike for MAP_LOCKED means that this would not be
useful.

> So, NAK.

V6 will not have the new mmap flag unless there is someone else that
speaks up in favor of keeping it.