btrfs: raid56: avoid double for loop inside __raid56_parity_recover()

The double for loop can be easily converted to single for loop as we're
really iterating the sectors in their bytenr order.

The only exception is the full stripe skip, however that can also easily
be done inside the loop.  Add an ASSERT() along with a comment for that
specific case.
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

@@ -2115,8 +2115,7 @@ static int __raid56_parity_recover(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);
@@ -2132,30 +2131,30 @@ static int __raid56_parity_recover(struct btrfs_raid_bio *rbio)
 	 * stripe cache, it is possible that some or all of these
 	 * pages are going to be uptodate.
 	 */
-	for (stripe = 0; stripe < rbio->real_stripes; stripe++) {
+	for (total_sector_nr = 0; total_sector_nr < rbio->nr_sectors;
+	     total_sector_nr++) {
+		int stripe = total_sector_nr / rbio->stripe_nsectors;
+		int sectornr = total_sector_nr % rbio->stripe_nsectors;
+		struct sector_ptr *sector;
+
 		if (rbio->faila == stripe || rbio->failb == stripe) {
 			atomic_inc(&rbio->error);
+			/* Skip the current stripe. */
+			ASSERT(sectornr == 0);
+			total_sector_nr += rbio->stripe_nsectors - 1;
 			continue;
 		}
-
-		for (sectornr = 0; sectornr < rbio->stripe_nsectors; sectornr++) {
-			struct sector_ptr *sector;
-
-			/*
-			 * the rmw code may have already read this
-			 * page in
-			 */
+		/* The RMW code may have already read this page in. */
 		sector = rbio_stripe_sector(rbio, stripe, sectornr);
 		if (sector->uptodate)
 			continue;
 
-			ret = rbio_add_io_sector(rbio, &bio_list, sector,
-						 stripe, sectornr, rbio->stripe_len,
+		ret = rbio_add_io_sector(rbio, &bio_list, sector, stripe,
+					 sectornr, rbio->stripe_len,
 					 REQ_OP_READ);
 		if (ret < 0)
 			goto cleanup;
 	}
-	}
 
 	bios_to_read = bio_list_size(&bio_list);
 	if (!bios_to_read) {