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

[Rik van Riel <riel@redhat.com>]

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?

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...

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