linux-kernel - Re: [PATCH V4 5/6] mm: mmap: Add mmap flag to request VM

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Date:	Wed, 22 Jul 2015 18:45:29 +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 V4 5/6] mm: mmap: Add mmap flag to request VM_LOCKONFAULT

On Wed, Jul 22, 2015 at 10:32:20AM -0400, Eric B Munson wrote:
> On Wed, 22 Jul 2015, Kirill A. Shutemov wrote:
> 
> > On Tue, Jul 21, 2015 at 03:59:40PM -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.
> > 
> > What is advantage over mmap() + mlock(MLOCK_ONFAULT)?
> 
> There isn't one, it was added to maintain parity with the
> mlock(MLOCK_LOCK) -> mmap(MAP_LOCKED) set.  I think not having will lead
> to confusion because we have MAP_LOCKED so why don't we support
> LOCKONFAULT from mmap as well.

I don't think it's ia good idea to spend bits in flags unless we have a
reason for that.

BTW, you have typo on sparc: s/0x8000/0x80000/.


-- 
 Kirill A. Shutemov
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