md: fix for divide error in status_resync

Stopping external metadata arrays during resync/recovery causes
retries, loop of interrupting and starting reconstruction, until it
hit at good moment to stop completely. While these retries
curr_mark_cnt can be small- especially on HDD drives, so subtraction
result can be smaller than 0. However it is casted to uint without
checking. As a result of it the status bar in /proc/mdstat while stopping
is strange (it jumps between 0% and 99%).

The real problem occurs here after commit 72deb455 ("block: remove
CONFIG_LBDAF"). Sector_div() macro has been changed, now the
divisor is casted to uint32. For db = -8 the divisior(db/32-1) becomes 0.

Check if db value can be really counted and replace these macro by
div64_u64() inline.
Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@intel.com>
Signed-off-by: Song Liu <songliubraving@fb.com>

drivers/md/md.c

@@ -7607,9 +7607,9 @@ static void status_unused(struct seq_file *seq)
 static int status_resync(struct seq_file *seq, struct mddev *mddev)
 {
 	sector_t max_sectors, resync, res;
-	unsigned long dt, db;
-	sector_t rt;
-	int scale;
+	unsigned long dt, db = 0;
+	sector_t rt, curr_mark_cnt, resync_mark_cnt;
+	int scale, recovery_active;
 	unsigned int per_milli;
 
 	if (test_bit(MD_RECOVERY_SYNC, &mddev->recovery) ||
@@ -7698,22 +7698,30 @@ static int status_resync(struct seq_file *seq, struct mddev *mddev)
 	 * db: blocks written from mark until now
 	 * rt: remaining time
 	 *
-	 * rt is a sector_t, so could be 32bit or 64bit.
-	 * So we divide before multiply in case it is 32bit and close
-	 * to the limit.
-	 * We scale the divisor (db) by 32 to avoid losing precision
-	 * near the end of resync when the number of remaining sectors
-	 * is close to 'db'.
-	 * We then divide rt by 32 after multiplying by db to compensate.
-	 * The '+1' avoids division by zero if db is very small.
+	 * rt is a sector_t, which is always 64bit now. We are keeping
+	 * the original algorithm, but it is not really necessary.
+	 *
+	 * Original algorithm:
+	 *   So we divide before multiply in case it is 32bit and close
+	 *   to the limit.
+	 *   We scale the divisor (db) by 32 to avoid losing precision
+	 *   near the end of resync when the number of remaining sectors
+	 *   is close to 'db'.
+	 *   We then divide rt by 32 after multiplying by db to compensate.
+	 *   The '+1' avoids division by zero if db is very small.
 	 */
 	dt = ((jiffies - mddev->resync_mark) / HZ);
 	if (!dt) dt++;
-	db = (mddev->curr_mark_cnt - atomic_read(&mddev->recovery_active))
-		- mddev->resync_mark_cnt;
+
+	curr_mark_cnt = mddev->curr_mark_cnt;
+	recovery_active = atomic_read(&mddev->recovery_active);
+	resync_mark_cnt = mddev->resync_mark_cnt;
+
+	if (curr_mark_cnt >= (recovery_active + resync_mark_cnt))
+		db = curr_mark_cnt - (recovery_active + resync_mark_cnt);
 
 	rt = max_sectors - resync;    /* number of remaining sectors */
-	sector_div(rt, db/32+1);
+	rt = div64_u64(rt, db/32+1);
 	rt *= dt;
 	rt >>= 5;