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

Kirill A. Shutemov
2015-07-22T15:45:37Z

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