drbd: fix false positive resync throttling in drbd_rs_c_min_rate_throttle

drivers/block/drbd/drbd_int.h

@@ -886,8 +886,6 @@ struct drbd_device {
 	atomic_t rs_sect_in; /* for incoming resync data rate, SyncTarget */
 	atomic_t rs_sect_ev; /* for submitted resync data rate, both */
 	int rs_last_sect_ev; /* counter to compare with */
-	int rs_last_events;  /* counter of read or write "events" (unit sectors)
-			      * on the lower level device when we last looked. */
 	int c_sync_rate; /* current resync rate after syncer throttle magic */
 	struct fifo_buffer *rs_plan_s; /* correction values of resync planer (RCU, connection->conn_update) */
 	int rs_in_flight; /* resync sectors in flight (to proxy, in proxy and from proxy) */

drivers/block/drbd/drbd_main.c

@@ -2018,7 +2018,6 @@ void drbd_device_cleanup(struct drbd_device *device)
 	device->rs_start     =
 	device->rs_total     =
 	device->rs_failed    = 0;
-	device->rs_last_events = 0;
 	device->rs_last_sect_ev = 0;
 	for (i = 0; i < DRBD_SYNC_MARKS; i++) {
 		device->rs_mark_left[i] = 0;

drivers/block/drbd/drbd_receiver.c

@@ -2569,50 +2569,56 @@ bool drbd_rs_should_slow_down(struct drbd_peer_device *peer_device, sector_t sec
 	return throttle;
 }
 
+/*
+ * Throttle resync when application I/O is in flight on the backing
+ * device and the resync is running faster than c-min-rate.
+ *
+ * The previous heuristic compared the backing device's part_stat
+ * sectors counter against our own rs_sect_ev counter (similar to
+ * MD RAID is_mddev_idle()) and fired on a delta > 64 sectors.  That
+ * comparison is racy: rs_sect_ev is bumped at submission while
+ * part_stat is updated on bio completion (and is per-cpu), and the
+ * receiver path bumps rs_sect_ev *after* the throttle check.  On
+ * fast hardware the transient skew routinely exceeds 64 sectors
+ * even with no application I/O, capping resync throughput at
+ * c-min-rate.
+ *
+ * ap_bio_cnt is incremented unconditionally for every application
+ * request in drbd_make_request(), so it is the authoritative
+ * "is application I/O in flight" signal.
+ */
 bool drbd_rs_c_min_rate_throttle(struct drbd_device *device)
 {
-	struct gendisk *disk = device->ldev->backing_bdev->bd_disk;
 	unsigned long db, dt, dbdt;
 	unsigned int c_min_rate;
-	int curr_events;
+	unsigned long rs_left;
+	int i;
 
 	rcu_read_lock();
 	c_min_rate = rcu_dereference(device->ldev->disk_conf)->c_min_rate;
 	rcu_read_unlock();
 
-	/* feature disabled? */
 	if (c_min_rate == 0)
 		return false;
 
-	curr_events = (int)part_stat_read_accum(disk->part0, sectors) -
-			atomic_read(&device->rs_sect_ev);
-
-	if (atomic_read(&device->ap_actlog_cnt)
-	    || curr_events - device->rs_last_events > 64) {
-		unsigned long rs_left;
-		int i;
-
-		device->rs_last_events = curr_events;
+	if (!atomic_read(&device->ap_bio_cnt))
+		return false;
 
-		/* sync speed average over the last 2*DRBD_SYNC_MARK_STEP,
-		 * approx. */
-		i = (device->rs_last_mark + DRBD_SYNC_MARKS-1) % DRBD_SYNC_MARKS;
+	/* sync speed average over the last 2*DRBD_SYNC_MARK_STEP, approx. */
+	i = (device->rs_last_mark + DRBD_SYNC_MARKS - 1) % DRBD_SYNC_MARKS;
 
-		if (device->state.conn == C_VERIFY_S || device->state.conn == C_VERIFY_T)
-			rs_left = device->ov_left;
-		else
-			rs_left = drbd_bm_total_weight(device) - device->rs_failed;
+	if (device->state.conn == C_VERIFY_S || device->state.conn == C_VERIFY_T)
+		rs_left = device->ov_left;
+	else
+		rs_left = drbd_bm_total_weight(device) - device->rs_failed;
 
-		dt = ((long)jiffies - (long)device->rs_mark_time[i]) / HZ;
-		if (!dt)
-			dt++;
-		db = device->rs_mark_left[i] - rs_left;
-		dbdt = Bit2KB(db/dt);
+	dt = ((long)jiffies - (long)device->rs_mark_time[i]) / HZ;
+	if (!dt)
+		dt++;
+	db = device->rs_mark_left[i] - rs_left;
+	dbdt = Bit2KB(db / dt);
 
-		if (dbdt > c_min_rate)
-			return true;
-	}
-	return false;
+	return dbdt > c_min_rate;
 }
 
 static int receive_DataRequest(struct drbd_connection *connection, struct packet_info *pi)

drivers/block/drbd/drbd_worker.c

@@ -1657,14 +1657,11 @@ void drbd_resync_after_changed(struct drbd_device *device)
 void drbd_rs_controller_reset(struct drbd_peer_device *peer_device)
 {
 	struct drbd_device *device = peer_device->device;
-	struct gendisk *disk = device->ldev->backing_bdev->bd_disk;
 	struct fifo_buffer *plan;
 
 	atomic_set(&device->rs_sect_in, 0);
 	atomic_set(&device->rs_sect_ev, 0);
 	device->rs_in_flight = 0;
-	device->rs_last_events =
-		(int)part_stat_read_accum(disk->part0, sectors);
 
 	/* Updating the RCU protected object in place is necessary since
 	   this function gets called from atomic context.