khugepaged: recheck pmd after mmap_sem re-acquired

Vlastimil noted[1] that pmd can be no longer valid after we drop
mmap_sem.  We need recheck it once mmap_sem taken again.

[1] http://lkml.kernel.org/r/12918dcd-a695-c6f4-e06f-69141c5f357f@suse.cz

Link: http://lkml.kernel.org/r/1466021202-61880-6-git-send-email-kirill.shutemov@linux.intel.com
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/huge_memory.c

@@ -2433,6 +2433,9 @@ static bool __collapse_huge_page_swapin(struct mm_struct *mm,
 			/* vma is no longer available, don't continue to swapin */
 			if (hugepage_vma_revalidate(mm, address))
 				return false;
+			/* check if the pmd is still valid */
+			if (mm_find_pmd(mm, address) != pmd)
+				return false;
 		}
 		if (ret & VM_FAULT_ERROR) {
 			trace_mm_collapse_huge_page_swapin(mm, swapped_in, 0);
@@ -2518,6 +2521,9 @@ static void collapse_huge_page(struct mm_struct *mm,
 	result = hugepage_vma_revalidate(mm, address);
 	if (result)
 		goto out;
+	/* check if the pmd is still valid */
+	if (mm_find_pmd(mm, address) != pmd)
+		goto out;
 
 	anon_vma_lock_write(vma->anon_vma);