[Devel] [PATCH rh7] mm/memcg: Bypass charges to offlined cgroup.

[Andrey Ryabinin]

Charges to offlined cgroup can happen. E.g. swapped
KSM page may charge to offlined cgroup. If such charge
races with reparenting during the offlining process
it may result in hang in mem_cgroup_reparent_charges()

Let's say we have cgroup A and cgroup B. A is parent of
B, B is offlined already, and A in the process of offlining.
During offline of A, we walk through all childs of A
and reparent their charges. After that, we reparent the A itself
If charge to B cgroup will appear after the reparenting
of B and before reparenting A, the reparanting of A will hang.
Charge to B also increases ->memory counter of its parent A,
so mem_cgroup_reparent_charges(A) will never satisfy the condition
'A->memory - A->kmem > 0'
which is required to break the loop.

https://jira.sw.ru/browse/PSBM-86092
Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
---
 mm/memcontrol.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 59cf47972f9e..8b979d88045c 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2909,6 +2909,28 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask, bool kmem_charge
 	return -EINTR;
 
 done_restock:
+
+	/*
+	 * Cancel charge in case if cgroup was offlined while we were here,
+	 * otherwise we can get a pending user memory charge to an offline
+	 * cgroup, which might race with reparanting in mem_cgroup_css_offline()
+	 * and result in hang.
+	 *
+	 * Note, no need to issue an explicit barrier here, because a
+	 * successful charge implies full memory barrier.
+	 */
+	if (unlikely(memcg->is_offline)) {
+		page_counter_uncharge(&memcg->memory, batch);
+		if (do_swap_account)
+			page_counter_uncharge(&memcg->memsw, batch);
+		if (cache_charge)
+			page_counter_uncharge(&memcg->cache, nr_pages);
+		if (kmem_charge)
+			page_counter_uncharge(&memcg->kmem, nr_pages);
+
+		goto bypass;
+	}
+
 	if (batch > nr_pages)
 		refill_stock(memcg, batch - nr_pages);
 done:
-- 
2.16.4