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>

1 files changed, 9 insertions, 7 deletions

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index a29c9d550391..2417f4fb8724 100644
--- a/fs/btrfs/volumes.c
+++ b/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;