Btrfs: Fix leaf reference cache miss

Due to the optimization for truncate, tree leaves only containing
checksum items can be deleted without being COW'ed first. This causes
reference cache misses. The way to fix the miss is create cache
entries for tree leaves only contain checksum.

This patch also fixes a -EEXIST issue in shared reference cache.
Signed-off-by: Yan Zheng <zheng.yan@oracle.com>

fs/btrfs/extent-tree.c

@@ -1151,6 +1151,14 @@ int btrfs_cache_ref(struct btrfs_trans_handle *trans, struct btrfs_root *root,
 		}
 
 		ret = btrfs_add_leaf_ref(root, ref, shared);
+		if (ret == -EEXIST && shared) {
+			struct btrfs_leaf_ref *old;
+			old = btrfs_lookup_leaf_ref(root, ref->bytenr);
+			BUG_ON(!old);
+			btrfs_remove_leaf_ref(root, old);
+			btrfs_free_leaf_ref(root, old);
+			ret = btrfs_add_leaf_ref(root, ref, shared);
+		}
 		WARN_ON(ret);
 		btrfs_free_leaf_ref(root, ref);
 	}

fs/btrfs/inode.c

@@ -48,6 +48,7 @@
 #include "xattr.h"
 #include "compat.h"
 #include "tree-log.h"
+#include "ref-cache.h"
 
 struct btrfs_iget_args {
 	u64 ino;
@@ -1416,6 +1417,9 @@ static noinline int drop_csum_leaves(struct btrfs_trans_handle *trans,
 	int nritems;
 	struct btrfs_key found_key;
 	struct btrfs_key other_key;
+	struct btrfs_leaf_ref *ref;
+	u64 leaf_gen;
+	u64 leaf_start;
 
 	path->lowest_level = 1;
 	key.objectid = inode->i_ino;
@@ -1509,15 +1513,31 @@ static noinline int drop_csum_leaves(struct btrfs_trans_handle *trans,
 	if (other_key.objectid != inode->i_ino || other_key.type != key.type)
 		goto out;
 
+	leaf_start = btrfs_node_blockptr(path->nodes[1], path->slots[1]);
+	leaf_gen = btrfs_node_ptr_generation(path->nodes[1], path->slots[1]);
 	/*
 	 * it is safe to delete this leaf, it contains only
 	 * csum items from this inode at an offset >= new_size
 	 */
-	ret = btrfs_del_leaf(trans, root, path,
-			     btrfs_node_blockptr(path->nodes[1],
-						 path->slots[1]));
+	ret = btrfs_del_leaf(trans, root, path, leaf_start);
 	BUG_ON(ret);
 
+	if (root->ref_cows && leaf_gen < trans->transid) {
+		ref = btrfs_alloc_leaf_ref(root, 0);
+		if (ref) {
+			ref->root_gen = root->root_key.offset;
+			ref->bytenr = leaf_start;
+			ref->owner = 0;
+			ref->generation = leaf_gen;
+			ref->nritems = 0;
+
+			ret = btrfs_add_leaf_ref(root, ref, 0);
+			WARN_ON(ret);
+			btrfs_free_leaf_ref(root, ref);
+		} else {
+			WARN_ON(1);
+		}
+	}
 next_key:
 	btrfs_release_path(root, path);