mm: fix swapoff hang after page migration and fork

I've been seeing swapoff hangs in recent testing: it's cycling around
trying unsuccessfully to find an mm for some remaining pages of swap.

I have been exercising swap and page migration more heavily recently,
and now notice a long-standing error in copy_one_pte(): it's trying to
add dst_mm to swapoff's mmlist when it finds a swap entry, but is doing
so even when it's a migration entry or an hwpoison entry.

Which wouldn't matter much, except it adds dst_mm next to src_mm,
assuming src_mm is already on the mmlist: which may not be so.  Then if
pages are later swapped out from dst_mm, swapoff won't be able to find
where to replace them.

There's already a !non_swap_entry() test for stats: move that up before
the swap_duplicate() and the addition to mmlist.
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Kelley Nielsen <kelleynnn@gmail.com>
Cc: <stable@vger.kernel.org>	[2.6.18+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory.c

@@ -815,6 +815,7 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 		if (!pte_file(pte)) {
 			swp_entry_t entry = pte_to_swp_entry(pte);
 
+			if (likely(!non_swap_entry(entry))) {
 				if (swap_duplicate(entry) < 0)
 					return entry.val;
 
@@ -826,9 +827,8 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 							 &src_mm->mmlist);
 					spin_unlock(&mmlist_lock);
 				}
-			if (likely(!non_swap_entry(entry)))
 				rss[MM_SWAPENTS]++;
-			else if (is_migration_entry(entry)) {
+			} else if (is_migration_entry(entry)) {
 				page = migration_entry_to_page(entry);
 
 				if (PageAnon(page))