btrfs: raid56: avoid double for loop inside raid56_parity_scrub_stripe()

Originally it's iterating all the sectors which has dbitmap sector for
the vertical stripe.

It can be easily converted to sector bytenr iteration with an test_bit()
call.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/raid56.c

@@ -2661,8 +2661,7 @@ static void raid56_parity_scrub_stripe(struct btrfs_raid_bio *rbio)
 	int bios_to_read = 0;
 	struct bio_list bio_list;
 	int ret;
-	int sectornr;
-	int stripe;
+	int total_sector_nr;
 	struct bio *bio;
 
 	bio_list_init(&bio_list);
@@ -2672,37 +2671,38 @@ static void raid56_parity_scrub_stripe(struct btrfs_raid_bio *rbio)
 		goto cleanup;
 
 	atomic_set(&rbio->error, 0);
-	/*
-	 * build a list of bios to read all the missing parts of this
-	 * stripe
-	 */
-	for (stripe = 0; stripe < rbio->real_stripes; stripe++) {
-		for_each_set_bit(sectornr, &rbio->dbitmap, rbio->stripe_nsectors) {
-			struct sector_ptr *sector;
-			/*
-			 * We want to find all the sectors missing from the
-			 * rbio and read them from the disk.  If * sector_in_rbio()
-			 * finds a sector in the bio list we don't need to read
-			 * it off the stripe.
-			 */
-			sector = sector_in_rbio(rbio, stripe, sectornr, 1);
-			if (sector)
-				continue;
+	/* Build a list of bios to read all the missing parts. */
+	for (total_sector_nr = 0; total_sector_nr < rbio->nr_sectors;
+	     total_sector_nr++) {
+		int sectornr = total_sector_nr % rbio->stripe_nsectors;
+		int stripe = total_sector_nr / rbio->stripe_nsectors;
+		struct sector_ptr *sector;
 
-			sector = rbio_stripe_sector(rbio, stripe, sectornr);
-			/*
-			 * The bio cache may have handed us an uptodate sector.
-			 * If so, be happy and use it.
-			 */
-			if (sector->uptodate)
-				continue;
+		/* No data in the vertical stripe, no need to read. */
+		if (!test_bit(sectornr, &rbio->dbitmap))
+			continue;
 
-			ret = rbio_add_io_sector(rbio, &bio_list, sector,
-						 stripe, sectornr, rbio->stripe_len,
-						 REQ_OP_READ);
-			if (ret)
-				goto cleanup;
-		}
+		/*
+		 * We want to find all the sectors missing from the rbio and
+		 * read them from the disk. If sector_in_rbio() finds a sector
+		 * in the bio list we don't need to read it off the stripe.
+		 */
+		sector = sector_in_rbio(rbio, stripe, sectornr, 1);
+		if (sector)
+			continue;
+
+		sector = rbio_stripe_sector(rbio, stripe, sectornr);
+		/*
+		 * The bio cache may have handed us an uptodate sector.  If so,
+		 * use it.
+		 */
+		if (sector->uptodate)
+			continue;
+
+		ret = rbio_add_io_sector(rbio, &bio_list, sector, stripe,
+					 sectornr, rbio->stripe_len, REQ_OP_READ);
+		if (ret)
+			goto cleanup;
 	}
 
 	bios_to_read = bio_list_size(&bio_list);