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

[Joonsoo Kim]

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?

Thanks.

> 
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