thp: add debug checks for mapcount related invariants

Add debug checks for invariants that if broken could lead to mapcount vs
page_mapcount debug checks to trigger later in split_huge_page.
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory.c

@@ -804,6 +804,7 @@ static inline int copy_pmd_range(struct mm_struct *dst_mm, struct mm_struct *src
 		next = pmd_addr_end(addr, end);
 		if (pmd_trans_huge(*src_pmd)) {
 			int err;
+			VM_BUG_ON(next-addr != HPAGE_PMD_SIZE);
 			err = copy_huge_pmd(dst_mm, src_mm,
 					    dst_pmd, src_pmd, addr, vma);
 			if (err == -ENOMEM)
@@ -1015,9 +1016,10 @@ static inline unsigned long zap_pmd_range(struct mmu_gather *tlb,
 	do {
 		next = pmd_addr_end(addr, end);
 		if (pmd_trans_huge(*pmd)) {
-			if (next-addr != HPAGE_PMD_SIZE)
+			if (next-addr != HPAGE_PMD_SIZE) {
+				VM_BUG_ON(!rwsem_is_locked(&tlb->mm->mmap_sem));
 				split_huge_page_pmd(vma->vm_mm, pmd);
-			else if (zap_huge_pmd(tlb, vma, pmd)) {
+			} else if (zap_huge_pmd(tlb, vma, pmd)) {
 				(*zap_work)--;
 				continue;
 			}