[PATCH RT] mm/memcg: Fix recursive locking on refill_stock() on PREEMPT_RT

[PATCH RT] mm/memcg: Fix recursive locking on refill_stock() on PREEMPT_RT

[Zachary Goldstein via B4 Relay]

From: Zachary Goldstein <zachary.goldstein@concurrent-rt.com>

5.10 suffers from the same recursive locking issue that
commit a848d25434de4 ("mm/memcg: Opencode the inner part of
obj_cgroup_uncharge_pages() in drain_obj_stock()") fixes.

Modified description from the commit to reflect this patch changes:

Provide the inner part of refill_stock() as __refill_stock() without
disabling interrupts. This eases the integration of local_lock_t where
recursive locking must be avoided.
Open code __memcg_kmem_uncharge() in drain_obj_stock() and
obj_cgroup_release() and use __refill_stock(). The caller of
drain_obj_stock() and obj_cgroup_release() already disables interrupts.

Signed-off-by: Zachary Goldstein <zachary.goldstein@concurrent-rt.com>
---
 mm/memcontrol.c | 28 +++++++++++++++++++++-------
 1 file changed, 21 insertions(+), 7 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 5dd77e260c25..d61918cc44c1 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -260,6 +260,8 @@ struct cgroup_subsys_state *vmpressure_to_css(struct vmpressure *vmpr)
 #ifdef CONFIG_MEMCG_KMEM
 static DEFINE_SPINLOCK(objcg_lock);
 
+static void __refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages);
+
 static void obj_cgroup_release(struct percpu_ref *ref)
 {
 	struct obj_cgroup *objcg = container_of(ref, struct obj_cgroup, refcnt);
@@ -294,8 +296,12 @@ static void obj_cgroup_release(struct percpu_ref *ref)
 
 	spin_lock_irqsave(&objcg_lock, flags);
 	memcg = obj_cgroup_memcg(objcg);
-	if (nr_pages)
-		__memcg_kmem_uncharge(memcg, nr_pages);
+	if (nr_pages) {
+		if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
+			page_counter_uncharge(&memcg->kmem, nr_pages);
+
+		__refill_stock(memcg, nr_pages);
+	}
 	list_del(&objcg->list);
 	mem_cgroup_put(memcg);
 	spin_unlock_irqrestore(&objcg_lock, flags);
@@ -2319,12 +2325,9 @@ static void drain_local_stock(struct work_struct *dummy)
  * Cache charges(val) to local per_cpu area.
  * This will be consumed by consume_stock() function, later.
  */
-static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
+static void __refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
 {
 	struct memcg_stock_pcp *stock;
-	unsigned long flags;
-
-	local_lock_irqsave(&memcg_stock.lock, flags);
 
 	stock = this_cpu_ptr(&memcg_stock);
 	if (stock->cached != memcg) { /* reset if necessary */
@@ -2336,7 +2339,14 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
 
 	if (stock->nr_pages > MEMCG_CHARGE_BATCH)
 		drain_stock(stock);
+}
 
+static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
+{
+	unsigned long flags;
+
+	local_lock_irqsave(&memcg_stock.lock, flags);
+	__refill_stock(memcg, nr_pages);
 	local_unlock_irqrestore(&memcg_stock.lock, flags);
 }
 
@@ -3179,7 +3189,11 @@ static void drain_obj_stock(struct memcg_stock_pcp *stock)
 				goto retry;
 			rcu_read_unlock();
 
-			__memcg_kmem_uncharge(memcg, nr_pages);
+			if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
+				page_counter_uncharge(&memcg->kmem, nr_pages);
+
+			__refill_stock(memcg, nr_pages);
+
 			css_put(&memcg->css);
 		}
 

--
2.39.2

Re: [PATCH RT] mm/memcg: Fix recursive locking on refill_stock() on PREEMPT_RT

[Sebastian Andrzej Siewior]

On 2023-08-10 15:05:34 [-0400], Zachary Goldstein via B4 Relay wrote:
> From: Zachary Goldstein <zachary.goldstein@concurrent-rt.com>
> 
> 5.10 suffers from the same recursive locking issue that
> commit a848d25434de4 ("mm/memcg: Opencode the inner part of
> obj_cgroup_uncharge_pages() in drain_obj_stock()") fixes.

This is the commit id from the stable-rt tree for the v5.15 series.
The original commit was af9a3b69e84be ("mm/memcg: opencode the inner
part of obj_cgroup_uncharge_pages() in drain_obj_stock()").

> Modified description from the commit to reflect this patch changes:
> 
> Provide the inner part of refill_stock() as __refill_stock() without
> disabling interrupts. This eases the integration of local_lock_t where
> recursive locking must be avoided.
> Open code __memcg_kmem_uncharge() in drain_obj_stock() and
> obj_cgroup_release() and use __refill_stock(). The caller of
> drain_obj_stock() and obj_cgroup_release() already disables interrupts.
> 
> Signed-off-by: Zachary Goldstein <zachary.goldstein@concurrent-rt.com>

I would prefer a proper backport similar to the commit you mentioned.
This would include keeping the author ship and so on.
The change itself looks okay.

Sebastian