[PATCH] mm: fix swapoff hang after page migration and fork

[Hugh Dickins <hughd@google.com>]

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: stable@vger.kernel.org # 2.6.18+
---

 mm/memory.c |   24 ++++++++++++------------
 1 file changed, 12 insertions(+), 12 deletions(-)

--- 3.18-rc7/mm/memory.c	2014-11-02 20:21:39.929011061 -0800
+++ linux/mm/memory.c	2014-11-27 19:38:52.314801502 -0800
@@ -815,20 +815,20 @@ copy_one_pte(struct mm_struct *dst_mm, s
 		if (!pte_file(pte)) {
 			swp_entry_t entry = pte_to_swp_entry(pte);
 
-			if (swap_duplicate(entry) < 0)
-				return entry.val;
+			if (likely(!non_swap_entry(entry))) {
+				if (swap_duplicate(entry) < 0)
+					return entry.val;
 
-			/* make sure dst_mm is on swapoff's mmlist. */
-			if (unlikely(list_empty(&dst_mm->mmlist))) {
-				spin_lock(&mmlist_lock);
-				if (list_empty(&dst_mm->mmlist))
-					list_add(&dst_mm->mmlist,
-						 &src_mm->mmlist);
-				spin_unlock(&mmlist_lock);
-			}
-			if (likely(!non_swap_entry(entry)))
+				/* make sure dst_mm is on swapoff's mmlist. */
+				if (unlikely(list_empty(&dst_mm->mmlist))) {
+					spin_lock(&mmlist_lock);
+					if (list_empty(&dst_mm->mmlist))
+						list_add(&dst_mm->mmlist,
+							 &src_mm->mmlist);
+					spin_unlock(&mmlist_lock);
+				}
 				rss[MM_SWAPENTS]++;
-			else if (is_migration_entry(entry)) {
+			} else if (is_migration_entry(entry)) {
 				page = migration_entry_to_page(entry);
 
 				if (PageAnon(page))

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