Btrfs: add and improve comments
[linux-2.6.git] / fs / btrfs / ref-cache.c
1 /*
2  * Copyright (C) 2008 Oracle.  All rights reserved.
3  *
4  * This program is free software; you can redistribute it and/or
5  * modify it under the terms of the GNU General Public
6  * License v2 as published by the Free Software Foundation.
7  *
8  * This program is distributed in the hope that it will be useful,
9  * but WITHOUT ANY WARRANTY; without even the implied warranty of
10  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
11  * General Public License for more details.
12  *
13  * You should have received a copy of the GNU General Public
14  * License along with this program; if not, write to the
15  * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
16  * Boston, MA 021110-1307, USA.
17  */
18
19 #include <linux/sched.h>
20 #include "ctree.h"
21 #include "ref-cache.h"
22 #include "transaction.h"
23
24 /*
25  * leaf refs are used to cache the information about which extents
26  * a given leaf has references on.  This allows us to process that leaf
27  * in btrfs_drop_snapshot without needing to read it back from disk.
28  */
29
30 /*
31  * kmalloc a leaf reference struct and update the counters for the
32  * total ref cache size
33  */
34 struct btrfs_leaf_ref *btrfs_alloc_leaf_ref(struct btrfs_root *root,
35                                             int nr_extents)
36 {
37         struct btrfs_leaf_ref *ref;
38         size_t size = btrfs_leaf_ref_size(nr_extents);
39
40         ref = kmalloc(size, GFP_NOFS);
41         if (ref) {
42                 spin_lock(&root->fs_info->ref_cache_lock);
43                 root->fs_info->total_ref_cache_size += size;
44                 spin_unlock(&root->fs_info->ref_cache_lock);
45
46                 memset(ref, 0, sizeof(*ref));
47                 atomic_set(&ref->usage, 1);
48                 INIT_LIST_HEAD(&ref->list);
49         }
50         return ref;
51 }
52
53 /*
54  * free a leaf reference struct and update the counters for the
55  * total ref cache size
56  */
57 void btrfs_free_leaf_ref(struct btrfs_root *root, struct btrfs_leaf_ref *ref)
58 {
59         if (!ref)
60                 return;
61         WARN_ON(atomic_read(&ref->usage) == 0);
62         if (atomic_dec_and_test(&ref->usage)) {
63                 size_t size = btrfs_leaf_ref_size(ref->nritems);
64
65                 BUG_ON(ref->in_tree);
66                 kfree(ref);
67
68                 spin_lock(&root->fs_info->ref_cache_lock);
69                 root->fs_info->total_ref_cache_size -= size;
70                 spin_unlock(&root->fs_info->ref_cache_lock);
71         }
72 }
73
74 static struct rb_node *tree_insert(struct rb_root *root, u64 bytenr,
75                                    struct rb_node *node)
76 {
77         struct rb_node ** p = &root->rb_node;
78         struct rb_node * parent = NULL;
79         struct btrfs_leaf_ref *entry;
80
81         while(*p) {
82                 parent = *p;
83                 entry = rb_entry(parent, struct btrfs_leaf_ref, rb_node);
84
85                 if (bytenr < entry->bytenr)
86                         p = &(*p)->rb_left;
87                 else if (bytenr > entry->bytenr)
88                         p = &(*p)->rb_right;
89                 else
90                         return parent;
91         }
92
93         entry = rb_entry(node, struct btrfs_leaf_ref, rb_node);
94         rb_link_node(node, parent, p);
95         rb_insert_color(node, root);
96         return NULL;
97 }
98
99 static struct rb_node *tree_search(struct rb_root *root, u64 bytenr)
100 {
101         struct rb_node * n = root->rb_node;
102         struct btrfs_leaf_ref *entry;
103
104         while(n) {
105                 entry = rb_entry(n, struct btrfs_leaf_ref, rb_node);
106                 WARN_ON(!entry->in_tree);
107
108                 if (bytenr < entry->bytenr)
109                         n = n->rb_left;
110                 else if (bytenr > entry->bytenr)
111                         n = n->rb_right;
112                 else
113                         return n;
114         }
115         return NULL;
116 }
117
118 int btrfs_remove_leaf_refs(struct btrfs_root *root, u64 max_root_gen,
119                            int shared)
120 {
121         struct btrfs_leaf_ref *ref = NULL;
122         struct btrfs_leaf_ref_tree *tree = root->ref_tree;
123
124         if (shared)
125                 tree = &root->fs_info->shared_ref_tree;
126         if (!tree)
127                 return 0;
128
129         spin_lock(&tree->lock);
130         while(!list_empty(&tree->list)) {
131                 ref = list_entry(tree->list.next, struct btrfs_leaf_ref, list);
132                 BUG_ON(ref->tree != tree);
133                 if (ref->root_gen > max_root_gen)
134                         break;
135                 if (!xchg(&ref->in_tree, 0)) {
136                         cond_resched_lock(&tree->lock);
137                         continue;
138                 }
139
140                 rb_erase(&ref->rb_node, &tree->root);
141                 list_del_init(&ref->list);
142
143                 spin_unlock(&tree->lock);
144                 btrfs_free_leaf_ref(root, ref);
145                 cond_resched();
146                 spin_lock(&tree->lock);
147         }
148         spin_unlock(&tree->lock);
149         return 0;
150 }
151
152 /*
153  * find the leaf ref for a given extent.  This returns the ref struct with
154  * a usage reference incremented
155  */
156 struct btrfs_leaf_ref *btrfs_lookup_leaf_ref(struct btrfs_root *root,
157                                              u64 bytenr)
158 {
159         struct rb_node *rb;
160         struct btrfs_leaf_ref *ref = NULL;
161         struct btrfs_leaf_ref_tree *tree = root->ref_tree;
162 again:
163         if (tree) {
164                 spin_lock(&tree->lock);
165                 rb = tree_search(&tree->root, bytenr);
166                 if (rb)
167                         ref = rb_entry(rb, struct btrfs_leaf_ref, rb_node);
168                 if (ref)
169                         atomic_inc(&ref->usage);
170                 spin_unlock(&tree->lock);
171                 if (ref)
172                         return ref;
173         }
174         if (tree != &root->fs_info->shared_ref_tree) {
175                 tree = &root->fs_info->shared_ref_tree;
176                 goto again;
177         }
178         return NULL;
179 }
180
181 /*
182  * add a fully filled in leaf ref struct
183  * remove all the refs older than a given root generation
184  */
185 int btrfs_add_leaf_ref(struct btrfs_root *root, struct btrfs_leaf_ref *ref,
186                        int shared)
187 {
188         int ret = 0;
189         struct rb_node *rb;
190         struct btrfs_leaf_ref_tree *tree = root->ref_tree;
191
192         if (shared)
193                 tree = &root->fs_info->shared_ref_tree;
194
195         spin_lock(&tree->lock);
196         rb = tree_insert(&tree->root, ref->bytenr, &ref->rb_node);
197         if (rb) {
198                 ret = -EEXIST;
199         } else {
200                 atomic_inc(&ref->usage);
201                 ref->tree = tree;
202                 ref->in_tree = 1;
203                 list_add_tail(&ref->list, &tree->list);
204         }
205         spin_unlock(&tree->lock);
206         return ret;
207 }
208
209 /*
210  * remove a single leaf ref from the tree.  This drops the ref held by the tree
211  * only
212  */
213 int btrfs_remove_leaf_ref(struct btrfs_root *root, struct btrfs_leaf_ref *ref)
214 {
215         struct btrfs_leaf_ref_tree *tree;
216
217         if (!xchg(&ref->in_tree, 0))
218                 return 0;
219
220         tree = ref->tree;
221         spin_lock(&tree->lock);
222
223         rb_erase(&ref->rb_node, &tree->root);
224         list_del_init(&ref->list);
225
226         spin_unlock(&tree->lock);
227
228         btrfs_free_leaf_ref(root, ref);
229         return 0;
230 }