linux-kernel - Re: [PATCH V5 5/7] mm: mmap: Add mmap flag to request VM

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Date:	Mon, 27 Jul 2015 17:03:55 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Eric B Munson <emunson@...mai.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>,
	Vlastimil Babka <vbabka@...e.cz>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Guenter Roeck <linux@...ck-us.net>,
	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 V5 5/7] mm: mmap: Add mmap flag to request VM_LOCKONFAULT

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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