btrfs: allocate backref_ctx on stack in find_extent_clone

Instead of using kmalloc() to allocate backref_ctx, allocate backref_ctx
on stack. The size is reasonably small.

sizeof(backref_ctx) = 48
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/send.c

@@ -1307,7 +1307,7 @@ static int find_extent_clone(struct send_ctx *sctx,
 	u64 flags = 0;
 	struct btrfs_file_extent_item *fi;
 	struct extent_buffer *eb = path->nodes[0];
-	struct backref_ctx *backref_ctx = NULL;
+	struct backref_ctx backref_ctx = {0};
 	struct clone_root *cur_clone_root;
 	struct btrfs_key found_key;
 	struct btrfs_path *tmp_path;
@@ -1322,12 +1322,6 @@ static int find_extent_clone(struct send_ctx *sctx,
 	/* We only use this path under the commit sem */
 	tmp_path->need_commit_sem = 0;
 
-	backref_ctx = kmalloc(sizeof(*backref_ctx), GFP_KERNEL);
-	if (!backref_ctx) {
-		ret = -ENOMEM;
-		goto out;
-	}
-
 	if (data_offset >= ino_size) {
 		/*
 		 * There may be extents that lie behind the file's size.
@@ -1392,12 +1386,12 @@ static int find_extent_clone(struct send_ctx *sctx,
 		cur_clone_root->found_refs = 0;
 	}
 
-	backref_ctx->sctx = sctx;
-	backref_ctx->found = 0;
-	backref_ctx->cur_objectid = ino;
-	backref_ctx->cur_offset = data_offset;
-	backref_ctx->found_itself = 0;
-	backref_ctx->extent_len = num_bytes;
+	backref_ctx.sctx = sctx;
+	backref_ctx.found = 0;
+	backref_ctx.cur_objectid = ino;
+	backref_ctx.cur_offset = data_offset;
+	backref_ctx.found_itself = 0;
+	backref_ctx.extent_len = num_bytes;
 
 	/*
 	 * The last extent of a file may be too large due to page alignment.
@@ -1405,7 +1399,7 @@ static int find_extent_clone(struct send_ctx *sctx,
 	 * __iterate_backrefs work.
 	 */
 	if (data_offset + num_bytes >= ino_size)
-		backref_ctx->extent_len = ino_size - data_offset;
+		backref_ctx.extent_len = ino_size - data_offset;
 
 	/*
 	 * Now collect all backrefs.
@@ -1416,12 +1410,12 @@ static int find_extent_clone(struct send_ctx *sctx,
 		extent_item_pos = 0;
 	ret = iterate_extent_inodes(fs_info, found_key.objectid,
 				    extent_item_pos, 1, __iterate_backrefs,
-				    backref_ctx, false);
+				    &backref_ctx, false);
 
 	if (ret < 0)
 		goto out;
 
-	if (!backref_ctx->found_itself) {
+	if (!backref_ctx.found_itself) {
 		/* found a bug in backref code? */
 		ret = -EIO;
 		btrfs_err(fs_info,
@@ -1434,7 +1428,7 @@ static int find_extent_clone(struct send_ctx *sctx,
 		    "find_extent_clone: data_offset=%llu, ino=%llu, num_bytes=%llu, logical=%llu",
 		    data_offset, ino, num_bytes, logical);
 
-	if (!backref_ctx->found)
+	if (!backref_ctx.found)
 		btrfs_debug(fs_info, "no clones found");
 
 	cur_clone_root = NULL;
@@ -1458,7 +1452,6 @@ static int find_extent_clone(struct send_ctx *sctx,
 
 out:
 	btrfs_free_path(tmp_path);
-	kfree(backref_ctx);
 	return ret;
 }