writeback: fix 100% CPU usage when dirtytime_expire_interval is 0

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>

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 6800886c4d1047b28c5c07613a1a1b256e982a95..cd21c74cd0e52358e5bb25acc29476fc116597f6 100644 (file)
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -2492,7 +2492,8 @@ 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 dirtytime_interval_handler(const struct ctl_table *table, int write,
@@ -2501,8 +2502,12 @@ static int dirtytime_interval_handler(const 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_percpu_wq, &dirtytime_work, 0);
+       if (ret == 0 && write) {
+               if (dirtytime_expire_interval)
+                       mod_delayed_work(system_percpu_wq, &dirtytime_work, 0);
+               else
+                       cancel_delayed_work_sync(&dirtytime_work);
+       }
        return ret;
 }
 
@@ -2519,7 +2524,8 @@ static const struct ctl_table vm_fs_writeback_table[] = {
 
 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);
        register_sysctl_init("vm", vm_fs_writeback_table);
        return 0;
 }