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

Wei Yang
2020-01-09T14:30:54Z

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@linux.intel.com>
Acked-by: David Rientjes <rientjes@google.com>

---
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