writeback: Fix wakeup and logging timeouts for !DETECT_HUNG_TASK

Recent changes of fs-writeback cause such warnings if DETECT_HUNG_TASK
is not enabled:

INFO: The task sync:1342 has been waiting for writeback completion for more than 1 seconds.

The reason is sysctl_hung_task_timeout_secs is 0 when DETECT_HUNG_TASK
is not enabled, then it causes the warning message even if the writeback
lasts for only one second.

Guard the wakeup and logging with "#ifdef CONFIG_DETECT_HUNG_TASK" can
eliminate the warning messages. But on the other hand, it is possible
that sysctl_hung_task_timeout_secs be also 0 when DETECT_HUNG_TASK is
enabled. So let's just check the value of sysctl_hung_task_timeout_secs
to decide whether do wakeup and logging.

Fixes: 1888635 ("writeback: Wake up waiting tasks when finishing the writeback of a chunk.")
Fixes: d6e6215 ("writeback: Add logging for slow writeback (exceeds sysctl_hung_task_timeout_secs)")
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Link: https://patch.msgid.link/20260203094014.2273240-1-chenhuacai@loongson.cn
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: chenhuacai

fs/fs-writeback.c

@@ -198,10 +198,11 @@ static void wb_queue_work(struct bdi_writeback *wb,
 
 static bool wb_wait_for_completion_cb(struct wb_completion *done)
 {
+	unsigned long timeout = sysctl_hung_task_timeout_secs;
 	unsigned long waited_secs = (jiffies - done->wait_start) / HZ;
 
 	done->progress_stamp = jiffies;
-	if (waited_secs > sysctl_hung_task_timeout_secs)
+	if (timeout && (waited_secs > timeout))
 		pr_info("INFO: The task %s:%d has been waiting for writeback "
 			"completion for more than %lu seconds.",
 			current->comm, current->pid, waited_secs);
@@ -1955,6 +1956,7 @@ static long writeback_sb_inodes(struct super_block *sb,
 		.range_end		= LLONG_MAX,
 	};
 	unsigned long start_time = jiffies;
+	unsigned long timeout = sysctl_hung_task_timeout_secs;
 	long write_chunk;
 	long total_wrote = 0;  /* count both pages and inodes */
 	unsigned long dirtied_before = jiffies;
@@ -2041,9 +2043,8 @@ static long writeback_sb_inodes(struct super_block *sb,
 		__writeback_single_inode(inode, &wbc);
 
 		/* Report progress to inform the hung task detector of the progress. */
-		if (work->done && work->done->progress_stamp &&
-		   (jiffies - work->done->progress_stamp) > HZ *
-		   sysctl_hung_task_timeout_secs / 2)
+		if (work->done && work->done->progress_stamp && timeout &&
+		   (jiffies - work->done->progress_stamp) > HZ * timeout / 2)
 			wake_up_all(work->done->waitq);
 
 		wbc_detach_inode(&wbc);