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Re: [PATCH V4 5/6] mm: mmap: Add mmap flag to request VM_LOCKONFAULT

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  Re: [PATCH V4 5/6] mm: mmap: Add mmap flag to request VM_LOCKONFAULT "Kirill A. Shutemov" <kirill@shutemov.name> - 2015-07-22 13:30 +0200
    Re: [PATCH V4 5/6] mm: mmap: Add mmap flag to request VM_LOCKONFAULT Eric B Munson <emunson@akamai.com> - 2015-07-22 16:40 +0200
      Re: [PATCH V4 5/6] mm: mmap: Add mmap flag to request VM_LOCKONFAULT "Kirill A. Shutemov" <kirill@shutemov.name> - 2015-07-22 17:50 +0200

#1189828 — Re: [PATCH V4 5/6] mm: mmap: Add mmap flag to request VM_LOCKONFAULT

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)?

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 Kirill A. Shutemov
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#1190019

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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.

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#1190072

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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