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

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  Re: [PATCH V5 5/7] mm: mmap: Add mmap flag to request VM_LOCKONFAULT Eric B Munson <emunson@akamai.com> - 2015-07-27 15:50 +0200
    Re: [PATCH V5 5/7] mm: mmap: Add mmap flag to request VM_LOCKONFAULT "Kirill A. Shutemov" <kirill@shutemov.name> - 2015-07-27 16:10 +0200
      Re: [PATCH V5 5/7] mm: mmap: Add mmap flag to request VM_LOCKONFAULT Eric B Munson <emunson@akamai.com> - 2015-07-27 16:20 +0200

#1193108 — Re: [PATCH V5 5/7] mm: mmap: Add mmap flag to request VM_LOCKONFAULT

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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/20150114095019.GC4706@dhcp22.suse.cz

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.

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#1193142

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/20150114095019.GC4706@dhcp22.suse.cz
> 
> 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.

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.

So, NAK.

-- 
 Kirill A. Shutemov
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#1193163

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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/20150114095019.GC4706@dhcp22.suse.cz
> > 
> > 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.

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