[PATCH 02/18] mm: memcontrol: fix theoretical race in charge moving

Johannes Weiner
2020-04-20T22:11:48Z

The move_lock is a per-memcg lock, but the VM accounting code that
needs to acquire it comes from the page and follows page->mem_cgroup
under RCU protection. That means that the page becomes unlocked not
when we drop the move_lock, but when we update page->mem_cgroup. And
that assignment doesn't imply any memory ordering. If that pointer
write gets reordered against the reads of the page state -
page_mapped, PageDirty etc. the state may change while we rely on it
being stable and we can end up corrupting the counters.

Place an SMP memory barrier to make sure we're done with all page
state by the time the new page->mem_cgroup becomes visible.

Also replace the open-coded move_lock with a lock_page_memcg() to make
it more obvious what we're serializing against.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
mm/memcontrol.c | 26 ++++++++++++++------------
1 file changed, 14 insertions(+), 12 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 5beea03dd58a..41f5ed79272e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5372,7 +5372,6 @@ static int mem_cgroup_move_account(struct page *page,
{
struct lruvec *from_vec, *to_vec;
struct pglist_data *pgdat;
- unsigned long flags;
unsigned int nr_pages = compound ? hpage_nr_pages(page) : 1;
int ret;
bool anon;
@@ -5399,18 +5398,13 @@ static int mem_cgroup_move_account(struct page *page,
from_vec = mem_cgroup_lruvec(from, pgdat);
to_vec = mem_cgroup_lruvec(to, pgdat);

- spin_lock_irqsave(&from->move_lock, flags);
+ lock_page_memcg(page);

if (!anon && page_mapped(page)) {
__mod_lruvec_state(from_vec, NR_FILE_MAPPED, -nr_pages);
__mod_lruvec_state(to_vec, NR_FILE_MAPPED, nr_pages);
}

- /*
- * move_lock grabbed above and caller set from->moving_account, so
- * mod_memcg_page_state will serialize updates to PageDirty.
- * So mapping should be stable for dirty pages.
- */
if (!anon && PageDirty(page)) {
struct address_space *mapping = page_mapping(page);

@@ -5426,15 +5420,23 @@ static int mem_cgroup_move_account(struct page *page,
}

/*
+ * All state has been migrated, let's switch to the new memcg.
+ *
* It is safe to change page->mem_cgroup here because the page
- * is referenced, charged, and isolated - we can't race with
- * uncharging, charging, migration, or LRU putback.
+ * is referenced, charged, isolated, and locked: we can't race
+ * with (un)charging, migration, LRU putback, or anything else
+ * that would rely on a stable page->mem_cgroup.
+ *
+ * Note that lock_page_memcg is a memcg lock, not a page lock,
+ * to save space. As soon as we switch page->mem_cgroup to a
+ * new memcg that isn't locked, the above state can change
+ * concurrently again. Make sure we're truly done with it.
*/
+ smp_mb();

- /* caller should have done css_get */
- page->mem_cgroup = to;
+ page->mem_cgroup = to; /* caller should have done css_get */

- spin_unlock_irqrestore(&from->move_lock, flags);
+ __unlock_page_memcg(from);

ret = 0;

--
2.26.0