The current charge path might race with memcg offlining because holding
css reference doesn't stop css offline. As a result res counter might be
charged after mem_cgroup_reparent_charges (called from memcg css_offline
callback) and so the charge would never be freed. This has been worked
around by 96f1c58d8534 (mm: memcg: fix race condition between memcg
teardown and swapin) which tries to catch such a leaked charges later
during css_free. It is more optimal to heal this race in the long term
though.
In order to make this raceless we would need to hold rcu_read_lock since
css_tryget until res_counter_charge. This is not so easy unfortunately
because mem_cgroup_do_charge might sleep so we would need to do drop rcu
lock and do css_tryget tricks after each reclaim.
This patch addresses the issue by introducing memcg->offline flag
which is set from mem_cgroup_css_offline callback before the pages are
reparented. mem_cgroup_do_charge checks the flag before res_counter
is charged inside rcu read section. mem_cgroup_css_offline uses
synchronize_rcu to let all preceding chargers finish while all the new
ones will see the group offline already and back out.
Callers are then updated to retry with a new memcg which is fallback to
mem_cgroup_from_task(current).
The only exception is mem_cgroup_do_precharge which should never see
this race because it is called from cgroup {can_}attach callbacks and so
the whole cgroup cannot go away.
Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
---
mm/memcontrol.c | 58 ++++++++++++++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 55 insertions(+), 3 deletions(-)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index a122fde6cd54..2904b2a6805a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -324,6 +324,9 @@ struct mem_cgroup {
int kmemcg_id;
#endif
+ /* Is memcg marked for offlining? */
+ bool offline;
+
int last_scanned_node;
#if MAX_NUMNODES > 1
nodemask_t scan_nodes;
@@ -2587,6 +2590,9 @@ enum {
CHARGE_RETRY, /* need to retry but retry is not bad */
CHARGE_NOMEM, /* we can't do more. return -ENOMEM */
CHARGE_WOULDBLOCK, /* GFP_WAIT wasn't set and no enough res. */
+ CHARGE_OFFLINE, /* memcg is offline already so no further
+ * charges are allowed
+ */
};
static int mem_cgroup_do_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
@@ -2599,20 +2605,36 @@ static int mem_cgroup_do_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
unsigned long flags = 0;
int ret;
+ /*
+ * Although the holder keeps a css reference for memcg this doesn't
+ * prevent it from being offlined in the meantime. We have to make sure
+ * that res counter is charged before css_offline reparents its pages
+ * otherwise the charge might leak.
+ */
+ rcu_read_lock();
+ if (memcg->offline) {
+ rcu_read_unlock();
+ return CHARGE_OFFLINE;
+ }
ret = res_counter_charge(&memcg->res, csize, &fail_res);
-
if (likely(!ret)) {
- if (!do_swap_account)
+ if (!do_swap_account) {
+ rcu_read_unlock();
return CHARGE_OK;
+ }
ret = res_counter_charge(&memcg->memsw, csize, &fail_res);
- if (likely(!ret))
+ if (likely(!ret)) {
+ rcu_read_unlock();
return CHARGE_OK;
+ }
res_counter_uncharge(&memcg->res, csize);
mem_over_limit = mem_cgroup_from_res_counter(fail_res, memsw);
flags |= MEM_CGROUP_RECLAIM_NOSWAP;
} else
mem_over_limit = mem_cgroup_from_res_counter(fail_res, res);
+ rcu_read_unlock();
+
/*
* Never reclaim on behalf of optional batching, retry with a
* single page instead.
@@ -2704,6 +2726,12 @@ static int __mem_cgroup_try_charge_memcg(gfp_t gfp_mask,
goto nomem;
nr_oom_retries--;
break;
+ /*
+ * memcg went offline, the caller should fallback to
+ * a different group.
+ */
+ case CHARGE_OFFLINE:
+ return -EAGAIN;
}
} while (ret != CHARGE_OK);
@@ -2747,6 +2775,7 @@ static struct mem_cgroup *mem_cgroup_try_charge_mm(struct mm_struct *mm,
VM_BUG_ON(!mm);
+again:
do {
if (mem_cgroup_bypass_charge())
goto bypass;
@@ -2778,6 +2807,8 @@ static struct mem_cgroup *mem_cgroup_try_charge_mm(struct mm_struct *mm,
goto bypass;
else if (ret == -ENOMEM)
memcg = NULL;
+ else if (ret == -EAGAIN)
+ goto again;
return memcg;
bypass:
@@ -3666,6 +3697,7 @@ __memcg_kmem_newpage_charge(gfp_t gfp, struct mem_cgroup **_memcg, int order)
if (!current->mm || current->memcg_kmem_skip_account)
return true;
+again:
memcg = try_get_mem_cgroup_from_mm(current->mm);
/*
@@ -3684,6 +3716,8 @@ __memcg_kmem_newpage_charge(gfp_t gfp, struct mem_cgroup **_memcg, int order)
ret = memcg_charge_kmem(memcg, gfp, PAGE_SIZE << order);
if (!ret)
*_memcg = memcg;
+ else if (ret == -EAGAIN)
+ goto again;
css_put(&memcg->css);
return (ret == 0);
@@ -4008,6 +4042,8 @@ static int __mem_cgroup_try_charge_swapin(struct mm_struct *mm,
if (ret == -EINTR) {
*memcgp = root_mem_cgroup;
ret = 0;
+ } else if (ret == -EAGAIN) {
+ goto charge_cur_mm;
}
return ret;
charge_cur_mm:
@@ -6340,6 +6376,14 @@ static void mem_cgroup_css_offline(struct cgroup_subsys_state *css)
{
struct mem_cgroup *memcg = mem_cgroup_from_css(css);
+ /*
+ * Mark the memcg offline and wait for the charger which uses
+ * RCU read lock to make sure no charges will leak.
+ * See mem_cgroup_do_charge for more details.
+ */
+ memcg->offline = true;
+ synchronize_rcu();
+
kmem_cgroup_css_offline(memcg);
mem_cgroup_invalidate_reclaim_iterators(memcg);
@@ -6437,6 +6481,14 @@ one_by_one:
cond_resched();
}
ret = mem_cgroup_try_charge_memcg(GFP_KERNEL, 1, memcg, false);
+
+ /*
+ * The target memcg cannot go offline because we are in
+ * move path and cgroup core doesn't allow to offline
+ * such groups.
+ */
+ BUG_ON(ret == -EAGAIN);
+
if (ret)
/* mem_cgroup_clear_mc() will do uncharge later */
return ret;
--
1.8.4.4
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