mm: numa: disable change protection for vma(VM_HUGETLB)

mm: numa: disable change protection for vma(VM_HUGETLB)

Currently when a process accesses a hugetlb range protected with PROTNONE,
unexpected COWs are triggered, which finally puts the hugetlb subsystem
into a broken/uncontrollable state, where for example h->resv_huge_pages
is subtracted too much and wraps around to a very large number, and the
free hugepage pool is no longer maintainable.

This patch simply stops changing protection for vma(VM_HUGETLB) to fix the
problem.  And this also allows us to avoid useless overhead of minor
faults.

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Suggested-by: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: David Rientjes <rientjes@google.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>