Hugetlbfs mmap with MAP_PRIVATE becomes MAP_SHARED silently, but
vma->vm_flags have no VM_SHARED bit.  Reading from /dev/zero into hugetlb
area will do:

read_zero()
    read_zero_pagealigned()
        if (vma->vm_flags & VM_SHARED)
            break;                      // fallback to clear_user()
        zap_page_range();
        zeromap_page_range();

It will hit BUG_ON() in unmap_hugepage_range() if region is not huge page
aligned, or silently convert it into the private anonymous mapping.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>