[PATCH 6.1.y] writeback: fix 100% CPU usage when dirtytime_expire_interval is 0

[Sasha Levin <sashal@kernel.org>]

From: Laveesh Bansal <laveeshb@laveeshbansal.com>

[ Upstream commit 543467d6fe97e27e22a26e367fda972dbefebbff ]

When vm.dirtytime_expire_seconds is set to 0, wakeup_dirtytime_writeback()
schedules delayed work with a delay of 0, causing immediate execution.
The function then reschedules itself with 0 delay again, creating an
infinite busy loop that causes 100% kworker CPU usage.

Fix by:
- Only scheduling delayed work in wakeup_dirtytime_writeback() when
  dirtytime_expire_interval is non-zero
- Cancelling the delayed work in dirtytime_interval_handler() when
  the interval is set to 0
- Adding a guard in start_dirtytime_writeback() for defensive coding

Tested by booting kernel in QEMU with virtme-ng:
- Before fix: kworker CPU spikes to ~73%
- After fix: CPU remains at normal levels
- Setting interval back to non-zero correctly resumes writeback

Fixes: a2f4870697a5 ("fs: make sure the timestamps for lazytime inodes eventually get written")
Cc: stable@vger.kernel.org
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=220227
Signed-off-by: Laveesh Bansal <laveeshb@laveeshbansal.com>
Link: https://patch.msgid.link/20260106145059.543282-2-laveeshb@laveeshbansal.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>
[ adapted system_percpu_wq to system_wq for the workqueue used in dirtytime_interval_handler() ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/fs-writeback.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 75e8c102c5eef..0a36fc5e1bf2c 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -2360,12 +2360,14 @@ static void wakeup_dirtytime_writeback(struct work_struct *w)
 				wb_wakeup(wb);
 	}
 	rcu_read_unlock();
-	schedule_delayed_work(&dirtytime_work, dirtytime_expire_interval * HZ);
+	if (dirtytime_expire_interval)
+		schedule_delayed_work(&dirtytime_work, dirtytime_expire_interval * HZ);
 }
 
 static int __init start_dirtytime_writeback(void)
 {
-	schedule_delayed_work(&dirtytime_work, dirtytime_expire_interval * HZ);
+	if (dirtytime_expire_interval)
+		schedule_delayed_work(&dirtytime_work, dirtytime_expire_interval * HZ);
 	return 0;
 }
 __initcall(start_dirtytime_writeback);
@@ -2376,8 +2378,12 @@ int dirtytime_interval_handler(struct ctl_table *table, int write,
 	int ret;
 
 	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
-	if (ret == 0 && write)
-		mod_delayed_work(system_wq, &dirtytime_work, 0);
+	if (ret == 0 && write) {
+		if (dirtytime_expire_interval)
+			mod_delayed_work(system_wq, &dirtytime_work, 0);
+		else
+			cancel_delayed_work_sync(&dirtytime_work);
+	}
 	return ret;
 }
 
-- 
2.51.0