[PATCH v2 06/10] btrfs: relocation: Use wrapper to replace open-coded edge linking

Qu Wenruo
2020-03-02T09:46:20Z

Since backref_edge is used to connect upper and lower backref nodes, and
need to access both nodes, some code can look pretty nasty:

list_add_tail(&edge->list[LOWER], &cur->upper);

The above code will link @cur to the LOWER side of the edge, while both
"LOWER" and "upper" words show up.
This can sometimes be very confusing for reader to grasp.

This patch introduce a new wrapper, link_backref_edge(), to handle the
linking behavior.
Which also has extra ASSERT() to ensure caller won't pass wrong nodes
in.

Also, this updates the comment of related lists of backref_node and
backref_edge, to make it more clear that each list points to what.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
fs/btrfs/relocation.c | 53 ++++++++++++++++++++++++++++++-------------
1 file changed, 37 insertions(+), 16 deletions(-)

diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index 04416489d87a..c76849409c81 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -91,10 +91,12 @@ struct backref_node {
u64 owner;
/* link to pending, changed or detached list */
struct list_head list;
- /* list of upper level blocks reference this block */
+
+ /* List of upper level edges, which links this node to its parent(s) */
struct list_head upper;
- /* list of child blocks in the cache */
+ /* List of lower level edges, which links this node to its child(ren) */
struct list_head lower;
+
/* NULL if this node is not tree root */
struct btrfs_root *root;
/* extent buffer got by COW the block */
@@ -123,17 +125,26 @@ struct backref_node {
unsigned int detached:1;
};

+#define LOWER 0
+#define UPPER 1
+#define RELOCATION_RESERVED_NODES 256
/*
- * present a block pointer in the backref cache
+ * present an edge connecting upper and lower backref nodes.
*/
struct backref_edge {
+ /*
+ * list[LOWER] is linked to backref_node::upper of lower level node,
+ * and list[UPPER] is linked to backref_node::lower of upper level node.
+ *
+ * Also, build_backref_tree() uses list[UPPER] for pending edges, before
+ * linking list[UPPER] to its upper level nodes.
+ */
struct list_head list[2];
+
+ /* Two related nodes */
struct backref_node *node[2];
};

-#define LOWER 0
-#define UPPER 1
-#define RELOCATION_RESERVED_NODES 256

struct backref_cache {
/* red black tree of all backref nodes in the cache */
@@ -332,6 +343,22 @@ static struct backref_edge *alloc_backref_edge(struct backref_cache *cache)
return edge;
}

+#define LINK_LOWER (1 << 0)
+#define LINK_UPPER (1 << 1)
+static inline void link_backref_edge(struct backref_edge *edge,
+ struct backref_node *lower,
+ struct backref_node *upper,
+ int link_which)
+{
+ ASSERT(upper && lower && upper->level == lower->level + 1);
+ edge->node[LOWER] = lower;
+ edge->node[UPPER] = upper;
+ if (link_which & LINK_LOWER)
+ list_add_tail(&edge->list[LOWER], &lower->upper);
+ if (link_which & LINK_UPPER)
+ list_add_tail(&edge->list[UPPER], &upper->lower);
+}
+
static void free_backref_edge(struct backref_cache *cache,
struct backref_edge *edge)
{
@@ -729,9 +756,7 @@ static int handle_one_tree_backref(struct reloc_control *rc,
ASSERT(upper->checked);
INIT_LIST_HEAD(&edge->list[UPPER]);
}
- list_add_tail(&edge->list[LOWER], &cur->upper);
- edge->node[LOWER] = cur;
- edge->node[UPPER] = upper;
+ link_backref_edge(edge, cur, upper, LINK_LOWER);
return 0;
}
/*
@@ -850,9 +875,7 @@ static int handle_one_tree_backref(struct reloc_control *rc,
if (!upper->owner)
upper->owner = btrfs_header_owner(eb);
}
- list_add_tail(&edge->list[LOWER], &lower->upper);
- edge->node[LOWER] = lower;
- edge->node[UPPER] = upper;
+ link_backref_edge(edge, lower, upper, LINK_LOWER);

if (rb_node) {
btrfs_put_root(root);
@@ -1246,10 +1269,8 @@ static int clone_backref_node(struct btrfs_trans_handle *trans,
if (!new_edge)
goto fail;

- new_edge->node[UPPER] = new_node;
- new_edge->node[LOWER] = edge->node[LOWER];
- list_add_tail(&new_edge->list[UPPER],
- &new_node->lower);
+ link_backref_edge(new_edge, edge->node[LOWER], new_node,
+ LINK_UPPER);
}
} else {
list_add_tail(&new_node->lower, &cache->leaves);
--
2.25.1