[RFC PATCH] memcontrol: Wait for draining of remote stocks to avoid OOM when charging

[RFC PATCH] memcontrol: Wait for draining of remote stocks to avoid OOM when charging

[Michal Koutný]

The LTP memcontrol03.c checks behavior of memory.min protection under
relatively tight conditions -- there is 2MiB margin for allocating task
below test's memory.max.

MEMCG_CHARGE_BATCH might be over-charged to page_counters temporarily
but this alone should not lead to OOM because this overcharged amount is
retrieved by draining stock. Or is it?

I suspect this may cause troubles when there is >MEMCG_CHARGE_BATCH charge
preceded by a small charge:

  try_charge_memcg(memcg, ..., 1);
    // counter->usage += 64
    // local stock = 63
    // no OOM but counter->usage > counter->max
  // running on different CPU
  try_charge_memcg(memcg, ..., 65);
    // 4M in stock + 148M new charge, only 150M w/out hard protection to reclaim
    try_to_free_mem_cgroup_pages
      if (cpu == curcpu)
        drain_local_stock // this would be ok
      else
        schedule_work_on(cpu, &stock->work); // this is asynchronous
      // charging+(no more)reclaim is retried MAX_RECLAIM_RETRIES = 16 times
      // if other cpu stock aren't flushed by now, this may cause OOM

This effect is pronounced on machines with 64k page size where it makes
MEMCG_CHARGE_BATCH worth whopping 4MiB (per CPU).

Prevent the premature OOM by waiting for stock flushing (even) from remote
CPUs.

Link: https://lore.kernel.org/ltp/144b6bac-edba-470a-bf87-abf492d85ef5@suse.cz/
Reported-by: Martin Doucha <mdoucha@suse.cz>
Signed-off-by: Michal Koutný <mkoutny@suse.com>
Tested-by: Martin Doucha <mdoucha@suse.cz>
---
 mm/memcontrol-v1.h |  2 +-
 mm/memcontrol.c    | 15 ++++++++++-----
 2 files changed, 11 insertions(+), 6 deletions(-)

My reason(s) for RFC:
1) I'm not sure if there isn't a simpler way than flushing stocks over
   all CPUs (also the guard with gfpflags_allow_blocking() is there only
   for explicitness, in case the code was moved over).
2) It requires specific scheduling over CPUs, so it may not be so common
   and severe in practice.

diff --git a/mm/memcontrol-v1.h b/mm/memcontrol-v1.h
index 6358464bb4160..3e57645d0c175 100644
--- a/mm/memcontrol-v1.h
+++ b/mm/memcontrol-v1.h
@@ -24,7 +24,7 @@
 
 unsigned long mem_cgroup_usage(struct mem_cgroup *memcg, bool swap);
 
-void drain_all_stock(struct mem_cgroup *root_memcg);
+void drain_all_stock(struct mem_cgroup *root_memcg, bool sync);
 
 unsigned long memcg_events(struct mem_cgroup *memcg, int event);
 unsigned long memcg_page_state_output(struct mem_cgroup *memcg, int item);
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 2d4d65f25fecd..ddf905baab12d 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1911,7 +1911,7 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
  * Drains all per-CPU charge caches for given root_memcg resp. subtree
  * of the hierarchy under it.
  */
-void drain_all_stock(struct mem_cgroup *root_memcg)
+void drain_all_stock(struct mem_cgroup *root_memcg, bool sync)
 {
 	int cpu, curcpu;
 
@@ -1948,6 +1948,11 @@ void drain_all_stock(struct mem_cgroup *root_memcg)
 				schedule_work_on(cpu, &stock->work);
 		}
 	}
+	if (sync)
+		for_each_online_cpu(cpu) {
+			struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
+			flush_work(&stock->work);
+		}
 	migrate_enable();
 	mutex_unlock(&percpu_charge_mutex);
 }
@@ -2307,7 +2312,7 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
 		goto retry;
 
 	if (!drained) {
-		drain_all_stock(mem_over_limit);
+		drain_all_stock(mem_over_limit, gfpflags_allow_blocking(gfp_mask));
 		drained = true;
 		goto retry;
 	}
@@ -3773,7 +3778,7 @@ static void mem_cgroup_css_offline(struct cgroup_subsys_state *css)
 	wb_memcg_offline(memcg);
 	lru_gen_offline_memcg(memcg);
 
-	drain_all_stock(memcg);
+	drain_all_stock(memcg, false);
 
 	mem_cgroup_id_put(memcg);
 }
@@ -4205,7 +4210,7 @@ static ssize_t memory_high_write(struct kernfs_open_file *of,
 			break;
 
 		if (!drained) {
-			drain_all_stock(memcg);
+			drain_all_stock(memcg, false);
 			drained = true;
 			continue;
 		}
@@ -4253,7 +4258,7 @@ static ssize_t memory_max_write(struct kernfs_open_file *of,
 			break;
 
 		if (!drained) {
-			drain_all_stock(memcg);
+			drain_all_stock(memcg, false);
 			drained = true;
 			continue;
 		}

base-commit: 0ff41df1cb268fc69e703a08a57ee14ae967d0ca
-- 
2.49.0

Re: [RFC PATCH] memcontrol: Wait for draining of remote stocks to avoid OOM when charging

[Michal Koutný]

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On Fri, May 30, 2025 at 05:18:57PM +0200, Michal Koutný <mkoutny@suse.com> wrote:
> 2) It requires specific scheduling over CPUs, so it may not be so common
>    and severe in practice.

This means in practice, there'd be likely a _different_ running memcg on
other CPUs and that would implicitly flush those stocks. I'm concluding
there's no big issue to fix.

Michal

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