[Patch v2] mm: thp: grab the lock before manipulation defer list

[Wei Yang]

As all the other places, we grab the lock before manipulate the defer list.
Current implementation may face a race condition.

For example, the potential race would be:

    CPU1                      CPU2
    mem_cgroup_move_account   split_huge_page_to_list
      !list_empty
                                lock
                                !list_empty
                                list_del
                                unlock
      lock
      # !list_empty might not hold anymore
      list_del_init
      unlock

When this sequence happens, the list_del_init() in
mem_cgroup_move_account() would crash if CONFIG_DEBUG_LIST since the
page is already been removed by list_del in split_huge_page_to_list().

Fixes: 87eaceb3faa5 ("mm: thp: make deferred split shrinker memcg aware")

Signed-off-by: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx>
Acked-by: David Rientjes <rientjes@xxxxxxxxxx>

---
v2:
  * move check on compound outside suggested by Alexander
  * an example of the race condition, suggested by Michal
---
 mm/memcontrol.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index bc01423277c5..1492eefe4f3c 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5368,10 +5368,12 @@ static int mem_cgroup_move_account(struct page *page,
 	}
 
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
-	if (compound && !list_empty(page_deferred_list(page))) {
+	if (compound) {
 		spin_lock(&from->deferred_split_queue.split_queue_lock);
-		list_del_init(page_deferred_list(page));
-		from->deferred_split_queue.split_queue_len--;
+		if (!list_empty(page_deferred_list(page))) {
+			list_del_init(page_deferred_list(page));
+			from->deferred_split_queue.split_queue_len--;
+		}
 		spin_unlock(&from->deferred_split_queue.split_queue_lock);
 	}
 #endif
@@ -5385,11 +5387,13 @@ static int mem_cgroup_move_account(struct page *page,
 	page->mem_cgroup = to;
 
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
-	if (compound && list_empty(page_deferred_list(page))) {
+	if (compound) {
 		spin_lock(&to->deferred_split_queue.split_queue_lock);
-		list_add_tail(page_deferred_list(page),
-			      &to->deferred_split_queue.split_queue);
-		to->deferred_split_queue.split_queue_len++;
+		if (list_empty(page_deferred_list(page))) {
+			list_add_tail(page_deferred_list(page),
+				      &to->deferred_split_queue.split_queue);
+			to->deferred_split_queue.split_queue_len++;
+		}
 		spin_unlock(&to->deferred_split_queue.split_queue_lock);
 	}
 #endif
-- 
2.17.1