FAILED: patch "[PATCH] mm/memcontrol: fix stuck FLUSHING_CACHED_CHARGE bit on isolated cpus" failed to apply to 6.6-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y
git checkout FETCH_HEAD
git cherry-pick -x d2f28a1a652e0a43f6f63de9b4e4d8d5f2716803
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261107053203-failed-6.6-d2f28a1a652e@kernel.org' --subject-prefix 'PATCH 6.6.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From d2f28a1a652e0a43f6f63de9b4e4d8d5f2716803 Mon Sep 17 00:00:00 2001
From: Rik van Riel <riel@surriel.com>
Date: Fri, 28 Aug 2026 13:50:36 -0400
Subject: [PATCH] mm/memcontrol: fix stuck FLUSHING_CACHED_CHARGE bit on
 isolated cpus

When drain_all_stock() sets FLUSHING_CACHED_CHARGE before checking
isolation, schedule_drain_work() can drop the work in a separate RCU
critical section, and housekeeping_update()'s synchronize_rcu() can race
that second check, leaving the flag set.

drain_local_stock() only clears the bit for work that ran, so the flag
remains set and the stock is never drained again.

Have schedule_drain_work() return whether the work was queued, and clear
FLUSHING_CACHED_CHARGE in drain_all_stock() when the remote CPU is
isolated, so future drains can retry.

Link: https://lore.kernel.org/20260828135036.7d44361f@fangorn
Fixes: 6a792697a53a ("memcg: do not drain charge pcp caches on remote isolated cpus")
Suggested-by: Michal Hocko <mhocko@suse.com>
Suggested-by: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: <stable@vger.kernel.org>
---
 mm/memcontrol.c | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 1709ac96bbdec..c6b85e3a5a0b3 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2306,7 +2306,7 @@ static bool is_memcg_drain_needed(struct memcg_stock_pcp *stock,
 	return flush;
 }
 
-static void schedule_drain_work(int cpu, struct work_struct *work)
+static bool schedule_drain_work(int cpu, struct work_struct *work)
 {
 	/*
 	 * Protect housekeeping cpumask read and work enqueue together
@@ -2315,8 +2315,11 @@ static void schedule_drain_work(int cpu, struct work_struct *work)
 	 * pending work on newly isolated CPUs.
 	 */
 	guard(rcu)();
-	if (!cpu_is_isolated(cpu))
-		queue_work_on(cpu, memcg_wq, work);
+	if (cpu_is_isolated(cpu))
+		return false;
+
+	queue_work_on(cpu, memcg_wq, work);
+	return true;
 }
 
 /*
@@ -2348,8 +2351,9 @@ void drain_all_stock(struct mem_cgroup *root_memcg)
 				      &memcg_st->flags)) {
 			if (cpu == curcpu)
 				drain_local_memcg_stock(&memcg_st->work);
-			else
-				schedule_drain_work(cpu, &memcg_st->work);
+			else if (!schedule_drain_work(cpu, &memcg_st->work))
+				clear_bit(FLUSHING_CACHED_CHARGE,
+					  &memcg_st->flags);
 		}
 
 		if (!test_bit(FLUSHING_CACHED_CHARGE, &obj_st->flags) &&
@@ -2358,8 +2362,9 @@ void drain_all_stock(struct mem_cgroup *root_memcg)
 				      &obj_st->flags)) {
 			if (cpu == curcpu)
 				drain_local_obj_stock(&obj_st->work);
-			else
-				schedule_drain_work(cpu, &obj_st->work);
+			else if (!schedule_drain_work(cpu, &obj_st->work))
+				clear_bit(FLUSHING_CACHED_CHARGE,
+					  &obj_st->flags);
 		}
 	}
 	migrate_enable();
-- 
2.53.0