btrfs: fold error checks when allocating ordered extent and update comments

Instead of having an error check and return on each branch of the if
statement, move the error check to happen after that if branch, reducing
source code and object code sizes.

Before this change:

   $ size fs/btrfs/btrfs.ko
      text	   data	    bss	    dec	    hex	filename
   1840174	 163742	  16136	2020052	 1ed2d4	fs/btrfs/btrfs.ko

After this change:

   $ size fs/btrfs/btrfs.ko
      text	   data	    bss	    dec	    hex	filename
   1840138	 163742	  16136	2020016	 1ed2b0	fs/btrfs/btrfs.ko

While at it and moving the comments, update the comments to be more clear
about how qgroup reserved space is released and the intricacies of how
it's managed for COW writes.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 14 insertions, 12 deletions

diff --git a/fs/btrfs/ordered-data.c b/fs/btrfs/ordered-data.c
index a6c5fc78bc4f..9212ce110cde 100644
--- a/fs/btrfs/ordered-data.c
+++ b/fs/btrfs/ordered-data.c
@@ -156,20 +156,22 @@ static struct btrfs_ordered_extent *alloc_ordered_extent(
 	const bool is_nocow = (flags &
 	       ((1U << BTRFS_ORDERED_NOCOW) | (1U << BTRFS_ORDERED_PREALLOC)));
 
-	if (is_nocow) {
-		/* For nocow write, we can release the qgroup rsv right now */
+	/*
+	 * For a NOCOW write we can free the qgroup reserve right now. For a COW
+	 * one we transfer the reserved space from the inode's iotree into the
+	 * ordered extent by calling btrfs_qgroup_release_data() and tracking
+	 * the qgroup reserved amount in the ordered extent, so that later after
+	 * completing the ordered extent, when running the data delayed ref it
+	 * creates, we free the reserved data with btrfs_qgroup_free_refroot().
+	 */
+	if (is_nocow)
 		ret = btrfs_qgroup_free_data(inode, NULL, file_offset, num_bytes, &qgroup_rsv);
-		if (ret < 0)
-			return ERR_PTR(ret);
-	} else {
-		/*
-		 * The ordered extent has reserved qgroup space, release now
-		 * and pass the reserved number for qgroup_record to free.
-		 */
+	else
 		ret = btrfs_qgroup_release_data(inode, file_offset, num_bytes, &qgroup_rsv);
-		if (ret < 0)
-			return ERR_PTR(ret);
-	}
+
+	if (ret < 0)
+		return ERR_PTR(ret);
+
 	entry = kmem_cache_zalloc(btrfs_ordered_extent_cache, GFP_NOFS);
 	if (!entry) {
 		entry = ERR_PTR(-ENOMEM);