btrfs: reduce type width of btrfs_io_contexts

That structure is our ultimate object for all __btrfs_map_block()
related functions.  We have some hard to understand members, like
tgtdev_map, but without any comments.

This patch will improve the situation:

- Add extra comments for num_stripes, mirror_num, num_tgtdevs and
  tgtdev_map[]
  Especially for the last two members, add a dedicated (thus very long)
  comments for them, with example to explain it.

- Shrink those int members to u16.
  In fact our on-disk format is only using u16 for num_stripes, thus
  no need to use int at all.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -5914,16 +5914,18 @@ static void sort_parity_stripes(struct btrfs_io_context *bioc, int num_stripes)
 }
 
 static struct btrfs_io_context *alloc_btrfs_io_context(struct btrfs_fs_info *fs_info,
-						       int total_stripes,
-						       int real_stripes)
+						       u16 total_stripes,
+						       u16 real_stripes)
 {
-	struct btrfs_io_context *bioc = kzalloc(
+	struct btrfs_io_context *bioc;
+
+	bioc = kzalloc(
 		 /* The size of btrfs_io_context */
 		sizeof(struct btrfs_io_context) +
 		/* Plus the variable array for the stripes */
 		sizeof(struct btrfs_io_stripe) * (total_stripes) +
 		/* Plus the variable array for the tgt dev */
-		sizeof(int) * (real_stripes) +
+		sizeof(u16) * (real_stripes) +
 		/*
 		 * Plus the raid_map, which includes both the tgt dev
 		 * and the stripes.
@@ -5937,7 +5939,7 @@ static struct btrfs_io_context *alloc_btrfs_io_context(struct btrfs_fs_info *fs_
 	refcount_set(&bioc->refs, 1);
 
 	bioc->fs_info = fs_info;
-	bioc->tgtdev_map = (int *)(bioc->stripes + total_stripes);
+	bioc->tgtdev_map = (u16 *)(bioc->stripes + total_stripes);
 	bioc->raid_map = (u64 *)(bioc->tgtdev_map + real_stripes);
 
 	return bioc;
@@ -6370,12 +6372,12 @@ int __btrfs_map_block(struct btrfs_fs_info *fs_info, enum btrfs_map_op op,
 	int mirror_num = (mirror_num_ret ? *mirror_num_ret : 0);
 	int num_stripes;
 	int max_errors = 0;
-	int tgtdev_indexes = 0;
 	struct btrfs_io_context *bioc = NULL;
 	struct btrfs_dev_replace *dev_replace = &fs_info->dev_replace;
 	int dev_replace_is_ongoing = 0;
-	int num_alloc_stripes;
 	int patch_the_first_stripe_for_dev_replace = 0;
+	u16 num_alloc_stripes;
+	u16 tgtdev_indexes = 0;
 	u64 physical_to_patch_in_first_stripe = 0;
 	u64 raid56_full_stripe_start = (u64)-1;
 	u64 max_len;

fs/btrfs/volumes.h

@@ -408,11 +408,55 @@ struct btrfs_io_context {
 	u64 map_type; /* get from map_lookup->type */
 	struct bio *orig_bio;
 	atomic_t error;
-	int max_errors;
-	int num_stripes;
-	int mirror_num;
-	int num_tgtdevs;
-	int *tgtdev_map;
+	u16 max_errors;
+
+	/*
+	 * The total number of stripes, including the extra duplicated
+	 * stripe for replace.
+	 */
+	u16 num_stripes;
+
+	/*
+	 * The mirror_num of this bioc.
+	 *
+	 * This is for reads which use 0 as mirror_num, thus we should return a
+	 * valid mirror_num (>0) for the reader.
+	 */
+	u16 mirror_num;
+
+	/*
+	 * The following two members are for dev-replace case only.
+	 *
+	 * @num_tgtdevs:	Number of duplicated stripes which need to be
+	 *			written to replace target.
+	 *			Should be <= 2 (2 for DUP, otherwise <= 1).
+	 * @tgtdev_map:		The array indicates where the duplicated stripes
+	 *			are from. The size is the number of original
+	 *			stripes (num_stripes - num_tgtdevs).
+	 *
+	 * The @tgtdev_map[] array is mostly for RAID56 cases.
+	 * As non-RAID56 stripes share the same contents of the mapped range,
+	 * thus no need to bother where the duplicated ones are from.
+	 *
+	 * But for RAID56 case, all stripes contain different contents, thus
+	 * we need a way to know the mapping.
+	 *
+	 * There is an example for the two members, using a RAID5 write:
+	 *
+	 *   num_stripes:	4 (3 + 1 duplicated write)
+	 *   stripes[0]:	dev = devid 1, physical = X
+	 *   stripes[1]:	dev = devid 2, physical = Y
+	 *   stripes[2]:	dev = devid 3, physical = Z
+	 *   stripes[3]:	dev = devid 0, physical = Y
+	 *
+	 * num_tgtdevs = 1
+	 * tgtdev_map[0] = 0	<- Means stripes[0] is not involved in replace.
+	 * tgtdev_map[1] = 3	<- Means stripes[1] is involved in replace,
+	 *			   and it's duplicated to stripes[3].
+	 * tgtdev_map[2] = 0	<- Means stripes[2] is not involved in replace.
+	 */
+	u16 num_tgtdevs;
+	u16 *tgtdev_map;
 	/*
 	 * logical block numbers for the start of each stripe
 	 * The last one or two are p/q.  These are sorted,