Re: [PATCH] mm/hugetlb: per-vma instantiation mutexes

[David Gibson <david@gibson.dropbear.id.au>]

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On Tue, Jul 16, 2013 at 02:34:24PM +0900, Joonsoo Kim wrote:
> On Mon, Jul 15, 2013 at 09:51:21PM -0400, Rik van Riel wrote:
> > On 07/15/2013 03:24 AM, David Gibson wrote:
> > >On Sun, Jul 14, 2013 at 08:16:44PM -0700, Davidlohr Bueso wrote:
> > 
> > >>>Reading the existing comment, this change looks very suspicious to me.
> > >>>A per-vma mutex is just not going to provide the necessary exclusion, is
> > >>>it?  (But I recall next to nothing about these regions and
> > >>>reservations.)
> > >
> > >A per-VMA lock is definitely wrong.  I think it handles one form of
> > >the race, between threads sharing a VM on a MAP_PRIVATE mapping.
> > >However another form of the race can and does occur between different
> > >MAP_SHARED VMAs in the same or different processes.  I think there may
> > >be edge cases involving mremap() and MAP_PRIVATE that will also be
> > >missed by a per-VMA lock.
> > >
> > >Note that the libhugetlbfs testsuite contains tests for both PRIVATE
> > >and SHARED variants of the race.
> > 
> > Can we get away with simply using a mutex in the file?
> > Say vma->vm_file->mapping->i_mmap_mutex?
> 
> I totally agree with this approach :)
> 
> > 
> > That might help with multiple processes initializing
> > multiple shared memory segments at the same time, and
> > should not hurt the case of a process mapping its own
> > hugetlbfs area.
> > 
> > It might have the potential to hurt when getting private
> > copies on a MAP_PRIVATE area, though.  I have no idea
> > how common it is for multiple processes to MAP_PRIVATE
> > the same hugetlbfs file, though...
> 
> Currently, getting private copies on a MAP_PRIVATE area is also
> serialized by hugetlb_instantiation_mutex.
> How do we get worse with your approach?
> 
> BTW, we have one race problem related to hugetlb_instantiation_mutex.
> It is not right protection for region structure handling. We map the
> area without holding a hugetlb_instantiation_mutex, so there is
> race condition between mapping a new area and faulting the other area.
> Am I missing?

The hugetlb_instantiation_mutex has nothing to do with protecting
region structures.  It exists only to address one very specific and
frequently misunderstood race.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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