Snapshot deletion requires walking down the btree and changing reference
counts of leaves and nodes. This caches the extents pointed to by a leaf
so that we can avoid reading that leaf during the snapshot deletion.
Many leaves don't point to any extents at all, so this is a huge win.
Signed-off-by: Chris Mason <chris.mason@xxxxxxxxxx>
diff --git a/fs/btrfs/ref-cache.c b/fs/btrfs/ref-cache.c
new file mode 100644
index 0000000..6f0acc4
--- /dev/null
+++ b/fs/btrfs/ref-cache.c
@@ -0,0 +1,230 @@
+/*
+ * Copyright (C) 2008 Oracle. All rights reserved.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public
+ * License v2 as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public
+ * License along with this program; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 021110-1307, USA.
+ */
+
+#include <linux/sched.h>
+#include "ctree.h"
+#include "ref-cache.h"
+#include "transaction.h"
+
+/*
+ * leaf refs are used to cache the information about which extents
+ * a given leaf has references on. This allows us to process that leaf
+ * in btrfs_drop_snapshot without needing to read it back from disk.
+ */
+
+/*
+ * kmalloc a leaf reference struct and update the counters for the
+ * total ref cache size
+ */
+struct btrfs_leaf_ref *btrfs_alloc_leaf_ref(struct btrfs_root *root,
+ int nr_extents)
+{
+ struct btrfs_leaf_ref *ref;
+ size_t size = btrfs_leaf_ref_size(nr_extents);
+
+ ref = kmalloc(size, GFP_NOFS);
+ if (ref) {
+ spin_lock(&root->fs_info->ref_cache_lock);
+ root->fs_info->total_ref_cache_size += size;
+ spin_unlock(&root->fs_info->ref_cache_lock);
+
+ memset(ref, 0, sizeof(*ref));
+ atomic_set(&ref->usage, 1);
+ INIT_LIST_HEAD(&ref->list);
+ }
+ return ref;
+}
+
+/*
+ * free a leaf reference struct and update the counters for the
+ * total ref cache size
+ */
+void btrfs_free_leaf_ref(struct btrfs_root *root, struct btrfs_leaf_ref *ref)
+{
+ if (!ref)
+ return;
+ WARN_ON(atomic_read(&ref->usage) == 0);
+ if (atomic_dec_and_test(&ref->usage)) {
+ size_t size = btrfs_leaf_ref_size(ref->nritems);
+
+ BUG_ON(ref->in_tree);
+ kfree(ref);
+
+ spin_lock(&root->fs_info->ref_cache_lock);
+ root->fs_info->total_ref_cache_size -= size;
+ spin_unlock(&root->fs_info->ref_cache_lock);
+ }
+}
+
+static struct rb_node *tree_insert(struct rb_root *root, u64 bytenr,
+ struct rb_node *node)
+{
+ struct rb_node **p = &root->rb_node;
+ struct rb_node *parent = NULL;
+ struct btrfs_leaf_ref *entry;
+
+ while (*p) {
+ parent = *p;
+ entry = rb_entry(parent, struct btrfs_leaf_ref, rb_node);
+
+ if (bytenr < entry->bytenr)
+ p = &(*p)->rb_left;
+ else if (bytenr > entry->bytenr)
+ p = &(*p)->rb_right;
+ else
+ return parent;
+ }
+
+ entry = rb_entry(node, struct btrfs_leaf_ref, rb_node);
+ rb_link_node(node, parent, p);
+ rb_insert_color(node, root);
+ return NULL;
+}
+
+static struct rb_node *tree_search(struct rb_root *root, u64 bytenr)
+{
+ struct rb_node *n = root->rb_node;
+ struct btrfs_leaf_ref *entry;
+
+ while (n) {
+ entry = rb_entry(n, struct btrfs_leaf_ref, rb_node);
+ WARN_ON(!entry->in_tree);
+
+ if (bytenr < entry->bytenr)
+ n = n->rb_left;
+ else if (bytenr > entry->bytenr)
+ n = n->rb_right;
+ else
+ return n;
+ }
+ return NULL;
+}
+
+int btrfs_remove_leaf_refs(struct btrfs_root *root, u64 max_root_gen,
+ int shared)
+{
+ struct btrfs_leaf_ref *ref = NULL;
+ struct btrfs_leaf_ref_tree *tree = root->ref_tree;
+
+ if (shared)
+ tree = &root->fs_info->shared_ref_tree;
+ if (!tree)
+ return 0;
+
+ spin_lock(&tree->lock);
+ while (!list_empty(&tree->list)) {
+ ref = list_entry(tree->list.next, struct btrfs_leaf_ref, list);
+ BUG_ON(ref->tree != tree);
+ if (ref->root_gen > max_root_gen)
+ break;
+ if (!xchg(&ref->in_tree, 0)) {
+ cond_resched_lock(&tree->lock);
+ continue;
+ }
+
+ rb_erase(&ref->rb_node, &tree->root);
+ list_del_init(&ref->list);
+
+ spin_unlock(&tree->lock);
+ btrfs_free_leaf_ref(root, ref);
+ cond_resched();
+ spin_lock(&tree->lock);
+ }
+ spin_unlock(&tree->lock);
+ return 0;
+}
+
+/*
+ * find the leaf ref for a given extent. This returns the ref struct with
+ * a usage reference incremented
+ */
+struct btrfs_leaf_ref *btrfs_lookup_leaf_ref(struct btrfs_root *root,
+ u64 bytenr)
+{
+ struct rb_node *rb;
+ struct btrfs_leaf_ref *ref = NULL;
+ struct btrfs_leaf_ref_tree *tree = root->ref_tree;
+again:
+ if (tree) {
+ spin_lock(&tree->lock);
+ rb = tree_search(&tree->root, bytenr);
+ if (rb)
+ ref = rb_entry(rb, struct btrfs_leaf_ref, rb_node);
+ if (ref)
+ atomic_inc(&ref->usage);
+ spin_unlock(&tree->lock);
+ if (ref)
+ return ref;
+ }
+ if (tree != &root->fs_info->shared_ref_tree) {
+ tree = &root->fs_info->shared_ref_tree;
+ goto again;
+ }
+ return NULL;
+}
+
+/*
+ * add a fully filled in leaf ref struct
+ * remove all the refs older than a given root generation
+ */
+int btrfs_add_leaf_ref(struct btrfs_root *root, struct btrfs_leaf_ref *ref,
+ int shared)
+{
+ int ret = 0;
+ struct rb_node *rb;
+ struct btrfs_leaf_ref_tree *tree = root->ref_tree;
+
+ if (shared)
+ tree = &root->fs_info->shared_ref_tree;
+
+ spin_lock(&tree->lock);
+ rb = tree_insert(&tree->root, ref->bytenr, &ref->rb_node);
+ if (rb) {
+ ret = -EEXIST;
+ } else {
+ atomic_inc(&ref->usage);
+ ref->tree = tree;
+ ref->in_tree = 1;
+ list_add_tail(&ref->list, &tree->list);
+ }
+ spin_unlock(&tree->lock);
+ return ret;
+}
+
+/*
+ * remove a single leaf ref from the tree. This drops the ref held by the tree
+ * only
+ */
+int btrfs_remove_leaf_ref(struct btrfs_root *root, struct btrfs_leaf_ref *ref)
+{
+ struct btrfs_leaf_ref_tree *tree;
+
+ if (!xchg(&ref->in_tree, 0))
+ return 0;
+
+ tree = ref->tree;
+ spin_lock(&tree->lock);
+
+ rb_erase(&ref->rb_node, &tree->root);
+ list_del_init(&ref->list);
+
+ spin_unlock(&tree->lock);
+
+ btrfs_free_leaf_ref(root, ref);
+ return 0;
+}
diff --git a/fs/btrfs/ref-cache.h b/fs/btrfs/ref-cache.h
new file mode 100644
index 0000000..16f3183
--- /dev/null
+++ b/fs/btrfs/ref-cache.h
@@ -0,0 +1,77 @@
+/*
+ * Copyright (C) 2008 Oracle. All rights reserved.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public
+ * License v2 as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public
+ * License along with this program; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 021110-1307, USA.
+ */
+#ifndef __REFCACHE__
+#define __REFCACHE__
+
+struct btrfs_extent_info {
+ /* bytenr and num_bytes find the extent in the extent allocation tree */
+ u64 bytenr;
+ u64 num_bytes;
+
+ /* objectid and offset find the back reference for the file */
+ u64 objectid;
+ u64 offset;
+};
+
+struct btrfs_leaf_ref {
+ struct rb_node rb_node;
+ struct btrfs_leaf_ref_tree *tree;
+ int in_tree;
+ atomic_t usage;
+
+ u64 root_gen;
+ u64 bytenr;
+ u64 owner;
+ u64 generation;
+ int nritems;
+
+ struct list_head list;
+ struct btrfs_extent_info extents[];
+};
+
+static inline size_t btrfs_leaf_ref_size(int nr_extents)
+{
+ return sizeof(struct btrfs_leaf_ref) +
+ sizeof(struct btrfs_extent_info) * nr_extents;
+}
+
+static inline void btrfs_leaf_ref_tree_init(struct btrfs_leaf_ref_tree *tree)
+{
+ tree->root.rb_node = NULL;
+ INIT_LIST_HEAD(&tree->list);
+ spin_lock_init(&tree->lock);
+}
+
+static inline int btrfs_leaf_ref_tree_empty(struct btrfs_leaf_ref_tree *tree)
+{
+ return RB_EMPTY_ROOT(&tree->root);
+}
+
+void btrfs_leaf_ref_tree_init(struct btrfs_leaf_ref_tree *tree);
+struct btrfs_leaf_ref *btrfs_alloc_leaf_ref(struct btrfs_root *root,
+ int nr_extents);
+void btrfs_free_leaf_ref(struct btrfs_root *root, struct btrfs_leaf_ref *ref);
+struct btrfs_leaf_ref *btrfs_lookup_leaf_ref(struct btrfs_root *root,
+ u64 bytenr);
+int btrfs_add_leaf_ref(struct btrfs_root *root, struct btrfs_leaf_ref *ref,
+ int shared);
+int btrfs_remove_leaf_refs(struct btrfs_root *root, u64 max_root_gen,
+ int shared);
+int btrfs_remove_leaf_ref(struct btrfs_root *root, struct btrfs_leaf_ref *ref);
+
+#endif
--
1.6.0.2
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