mm/swapfile: skip HugeTLB pages for unuse_vma

I got a bad pud error and lost a 1GB HugeTLB when calling swapoff.  The
problem can be reproduced by the following steps:

 1. Allocate an anonymous 1GB HugeTLB and some other anonymous memory.
 2. Swapout the above anonymous memory.
 3. run swapoff and we will get a bad pud error in kernel message:

  mm/pgtable-generic.c:42: bad pud 00000000743d215d(84000001400000e7)

We can tell that pud_clear_bad is called by pud_none_or_clear_bad in
unuse_pud_range() by ftrace.  And therefore the HugeTLB pages will never
be freed because we lost it from page table.  We can skip HugeTLB pages
for unuse_vma to fix it.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 0fe6e20b9c ("hugetlb, rmap: add reverse mapping for hugepage")
Signed-off-by: Liu Shixin <[email protected]>
Acked-by: Muchun Song <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/swapfile.c

@@ -2313,7 +2313,7 @@ static int unuse_mm(struct mm_struct *mm, unsigned int type)
 
 	mmap_read_lock(mm);
 	for_each_vma(vmi, vma) {
-		if (vma->anon_vma) {
+		if (vma->anon_vma && !is_vm_hugetlb_page(vma)) {
 			ret = unuse_vma(vma, type);
 			if (ret)
 				break;