[PATCH 6.6.y] mm/memcontrol: fix stuck FLUSHING_CACHED_CHARGE bit on isolated cpus

[Sasha Levin <sashal@kernel.org>]

From: Rik van Riel <riel@surriel.com>

[ Upstream commit d2f28a1a652e0a43f6f63de9b4e4d8d5f2716803 ]

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>
[ sashal: adapted schedule_drain_work() return handling to a direct flag clear in the existing single-stock isolation branch ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 mm/memcontrol.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 852fa93efe1be..d31ceaeb13b05 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2396,6 +2396,8 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
 				drain_local_stock(&stock->work);
 			else if (!cpu_is_isolated(cpu))
 				schedule_work_on(cpu, &stock->work);
+			else
+				clear_bit(FLUSHING_CACHED_CHARGE, &stock->flags);
 		}
 	}
 	migrate_enable();
-- 
2.53.0