btrfs: simplify extent map splitting and rename split_zoned_em

split_zoned_em is only ever asked to split out the beginning of an extent
map.  Change it to only take a len to split out instead of a pre and post
region.

Also rename the function to split_extent_map as there is nothing zoned
device specific about it.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Tested-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -2512,37 +2512,31 @@ void btrfs_clear_delalloc_extent(struct btrfs_inode *inode,
 }
 
 /*
- * Split an extent_map at [start, start + len]
+ * Split off the first pre bytes from the extent_map at [start, start + len]
  *
  * This function is intended to be used only for extract_ordered_extent().
  */
-static int split_zoned_em(struct btrfs_inode *inode, u64 start, u64 len,
-			  u64 pre, u64 post)
+static int split_extent_map(struct btrfs_inode *inode, u64 start, u64 len, u64 pre)
 {
 	struct extent_map_tree *em_tree = &inode->extent_tree;
 	struct extent_map *em;
 	struct extent_map *split_pre = NULL;
 	struct extent_map *split_mid = NULL;
-	struct extent_map *split_post = NULL;
 	int ret = 0;
 	unsigned long flags;
 
-	/* Sanity check */
-	if (pre == 0 && post == 0)
-		return 0;
+	ASSERT(pre != 0);
+	ASSERT(pre < len);
 
 	split_pre = alloc_extent_map();
-	if (pre)
-		split_mid = alloc_extent_map();
-	if (post)
-		split_post = alloc_extent_map();
-	if (!split_pre || (pre && !split_mid) || (post && !split_post)) {
+	if (!split_pre)
+		return -ENOMEM;
+	split_mid = alloc_extent_map();
+	if (!split_mid) {
 		ret = -ENOMEM;
-		goto out;
+		goto out_free_pre;
 	}
 
-	ASSERT(pre + post < len);
-
 	lock_extent(&inode->io_tree, start, start + len - 1, NULL);
 	write_lock(&em_tree->lock);
 	em = lookup_extent_mapping(em_tree, start, len);
@@ -2563,7 +2557,7 @@ static int split_zoned_em(struct btrfs_inode *inode, u64 start, u64 len,
 
 	/* First, replace the em with a new extent_map starting from * em->start */
 	split_pre->start = em->start;
-	split_pre->len = (pre ? pre : em->len - post);
+	split_pre->len = pre;
 	split_pre->orig_start = split_pre->start;
 	split_pre->block_start = em->block_start;
 	split_pre->block_len = split_pre->len;
@@ -2577,38 +2571,21 @@ static int split_zoned_em(struct btrfs_inode *inode, u64 start, u64 len,
 
 	/*
 	 * Now we only have an extent_map at:
-	 *     [em->start, em->start + pre] if pre != 0
-	 *     [em->start, em->start + em->len - post] if pre == 0
+	 *     [em->start, em->start + pre]
 	 */
 
-	if (pre) {
-		/* Insert the middle extent_map */
-		split_mid->start = em->start + pre;
-		split_mid->len = em->len - pre - post;
-		split_mid->orig_start = split_mid->start;
-		split_mid->block_start = em->block_start + pre;
-		split_mid->block_len = split_mid->len;
-		split_mid->orig_block_len = split_mid->block_len;
-		split_mid->ram_bytes = split_mid->len;
-		split_mid->flags = flags;
-		split_mid->compress_type = em->compress_type;
-		split_mid->generation = em->generation;
-		add_extent_mapping(em_tree, split_mid, 1);
-	}
-
-	if (post) {
-		split_post->start = em->start + em->len - post;
-		split_post->len = post;
-		split_post->orig_start = split_post->start;
-		split_post->block_start = em->block_start + em->len - post;
-		split_post->block_len = split_post->len;
-		split_post->orig_block_len = split_post->block_len;
-		split_post->ram_bytes = split_post->len;
-		split_post->flags = flags;
-		split_post->compress_type = em->compress_type;
-		split_post->generation = em->generation;
-		add_extent_mapping(em_tree, split_post, 1);
-	}
+	/* Insert the middle extent_map. */
+	split_mid->start = em->start + pre;
+	split_mid->len = em->len - pre;
+	split_mid->orig_start = split_mid->start;
+	split_mid->block_start = em->block_start + pre;
+	split_mid->block_len = split_mid->len;
+	split_mid->orig_block_len = split_mid->block_len;
+	split_mid->ram_bytes = split_mid->len;
+	split_mid->flags = flags;
+	split_mid->compress_type = em->compress_type;
+	split_mid->generation = em->generation;
+	add_extent_mapping(em_tree, split_mid, 1);
 
 	/* Once for us */
 	free_extent_map(em);
@@ -2618,11 +2595,9 @@ static int split_zoned_em(struct btrfs_inode *inode, u64 start, u64 len,
 out_unlock:
 	write_unlock(&em_tree->lock);
 	unlock_extent(&inode->io_tree, start, start + len - 1, NULL);
-out:
-	free_extent_map(split_pre);
 	free_extent_map(split_mid);
-	free_extent_map(split_post);
-
+out_free_pre:
+	free_extent_map(split_pre);
 	return ret;
 }
 
@@ -2653,7 +2628,7 @@ blk_status_t btrfs_extract_ordered_extent(struct btrfs_bio *bbio)
 	ret = btrfs_split_ordered_extent(ordered, len);
 	if (ret)
 		goto out;
-	ret = split_zoned_em(inode, bbio->file_offset, ordered_len, len, 0);
+	ret = split_extent_map(inode, bbio->file_offset, ordered_len, len);
 out:
 	btrfs_put_ordered_extent(ordered);
 	return errno_to_blk_status(ret);