Revert "mm: do not collapse stack gap into THP"

This reverts commit cc9020f7 to be
replaced by the upstream patch set.

CVE-2017-1000364
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

mm/huge_memory.c

@@ -815,9 +815,6 @@ int do_huge_pmd_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
 
 	if (haddr < vma->vm_start || haddr + HPAGE_PMD_SIZE > vma->vm_end)
 		return VM_FAULT_FALLBACK;
-	if (stack_guard_area(vma, haddr) ||
-			stack_guard_area(vma, haddr + HPAGE_PMD_SIZE))
-		return VM_FAULT_FALLBACK;
 	if (unlikely(anon_vma_prepare(vma)))
 		return VM_FAULT_OOM;
 	if (unlikely(khugepaged_enter(vma, vma->vm_flags)))
@@ -2564,9 +2561,6 @@ static void collapse_huge_page(struct mm_struct *mm,
 		goto out;
 	if (!hugepage_vma_check(vma))
 		goto out;
-	/* never try to collapse stack gap */
-	if (stack_guard_area(vma, hstart) || stack_guard_area(vma, hend))
-		goto out;
 	pmd = mm_find_pmd(mm, address);
 	if (!pmd)
 		goto out;