1 | /* |
2 | * QEMU Enhanced Disk Format L2 Cache |
3 | * |
4 | * Copyright IBM, Corp. 2010 |
5 | * |
6 | * Authors: |
7 | * Anthony Liguori <aliguori@us.ibm.com> |
8 | * |
9 | * This work is licensed under the terms of the GNU LGPL, version 2 or later. |
10 | * See the COPYING.LIB file in the top-level directory. |
11 | * |
12 | */ |
13 | |
14 | /* |
15 | * L2 table cache usage is as follows: |
16 | * |
17 | * An open image has one L2 table cache that is used to avoid accessing the |
18 | * image file for recently referenced L2 tables. |
19 | * |
20 | * Cluster offset lookup translates the logical offset within the block device |
21 | * to a cluster offset within the image file. This is done by indexing into |
22 | * the L1 and L2 tables which store cluster offsets. It is here where the L2 |
23 | * table cache serves up recently referenced L2 tables. |
24 | * |
25 | * If there is a cache miss, that L2 table is read from the image file and |
26 | * committed to the cache. Subsequent accesses to that L2 table will be served |
27 | * from the cache until the table is evicted from the cache. |
28 | * |
29 | * L2 tables are also committed to the cache when new L2 tables are allocated |
30 | * in the image file. Since the L2 table cache is write-through, the new L2 |
31 | * table is first written out to the image file and then committed to the |
32 | * cache. |
33 | * |
34 | * Multiple I/O requests may be using an L2 table cache entry at any given |
35 | * time. That means an entry may be in use across several requests and |
36 | * reference counting is needed to free the entry at the correct time. In |
37 | * particular, an entry evicted from the cache will only be freed once all |
38 | * references are dropped. |
39 | * |
40 | * An in-flight I/O request will hold a reference to a L2 table cache entry for |
41 | * the period during which it needs to access the L2 table. This includes |
42 | * cluster offset lookup, L2 table allocation, and L2 table update when a new |
43 | * data cluster has been allocated. |
44 | * |
45 | * An interesting case occurs when two requests need to access an L2 table that |
46 | * is not in the cache. Since the operation to read the table from the image |
47 | * file takes some time to complete, both requests may see a cache miss and |
48 | * start reading the L2 table from the image file. The first to finish will |
49 | * commit its L2 table into the cache. When the second tries to commit its |
50 | * table will be deleted in favor of the existing cache entry. |
51 | */ |
52 | |
53 | #include "qemu/osdep.h" |
54 | #include "trace.h" |
55 | #include "qed.h" |
56 | |
57 | /* Each L2 holds 2GB so this let's us fully cache a 100GB disk */ |
58 | #define MAX_L2_CACHE_SIZE 50 |
59 | |
60 | /** |
61 | * Initialize the L2 cache |
62 | */ |
63 | void qed_init_l2_cache(L2TableCache *l2_cache) |
64 | { |
65 | QTAILQ_INIT(&l2_cache->entries); |
66 | l2_cache->n_entries = 0; |
67 | } |
68 | |
69 | /** |
70 | * Free the L2 cache |
71 | */ |
72 | void qed_free_l2_cache(L2TableCache *l2_cache) |
73 | { |
74 | CachedL2Table *entry, *next_entry; |
75 | |
76 | QTAILQ_FOREACH_SAFE(entry, &l2_cache->entries, node, next_entry) { |
77 | qemu_vfree(entry->table); |
78 | g_free(entry); |
79 | } |
80 | } |
81 | |
82 | /** |
83 | * Allocate an uninitialized entry from the cache |
84 | * |
85 | * The returned entry has a reference count of 1 and is owned by the caller. |
86 | * The caller must allocate the actual table field for this entry and it must |
87 | * be freeable using qemu_vfree(). |
88 | */ |
89 | CachedL2Table *qed_alloc_l2_cache_entry(L2TableCache *l2_cache) |
90 | { |
91 | CachedL2Table *entry; |
92 | |
93 | entry = g_malloc0(sizeof(*entry)); |
94 | entry->ref++; |
95 | |
96 | trace_qed_alloc_l2_cache_entry(l2_cache, entry); |
97 | |
98 | return entry; |
99 | } |
100 | |
101 | /** |
102 | * Decrease an entry's reference count and free if necessary when the reference |
103 | * count drops to zero. |
104 | * |
105 | * Called with table_lock held. |
106 | */ |
107 | void qed_unref_l2_cache_entry(CachedL2Table *entry) |
108 | { |
109 | if (!entry) { |
110 | return; |
111 | } |
112 | |
113 | entry->ref--; |
114 | trace_qed_unref_l2_cache_entry(entry, entry->ref); |
115 | if (entry->ref == 0) { |
116 | qemu_vfree(entry->table); |
117 | g_free(entry); |
118 | } |
119 | } |
120 | |
121 | /** |
122 | * Find an entry in the L2 cache. This may return NULL and it's up to the |
123 | * caller to satisfy the cache miss. |
124 | * |
125 | * For a cached entry, this function increases the reference count and returns |
126 | * the entry. |
127 | * |
128 | * Called with table_lock held. |
129 | */ |
130 | CachedL2Table *qed_find_l2_cache_entry(L2TableCache *l2_cache, uint64_t offset) |
131 | { |
132 | CachedL2Table *entry; |
133 | |
134 | QTAILQ_FOREACH(entry, &l2_cache->entries, node) { |
135 | if (entry->offset == offset) { |
136 | trace_qed_find_l2_cache_entry(l2_cache, entry, offset, entry->ref); |
137 | entry->ref++; |
138 | return entry; |
139 | } |
140 | } |
141 | return NULL; |
142 | } |
143 | |
144 | /** |
145 | * Commit an L2 cache entry into the cache. This is meant to be used as part of |
146 | * the process to satisfy a cache miss. A caller would allocate an entry which |
147 | * is not actually in the L2 cache and then once the entry was valid and |
148 | * present on disk, the entry can be committed into the cache. |
149 | * |
150 | * Since the cache is write-through, it's important that this function is not |
151 | * called until the entry is present on disk and the L1 has been updated to |
152 | * point to the entry. |
153 | * |
154 | * N.B. This function steals a reference to the l2_table from the caller so the |
155 | * caller must obtain a new reference by issuing a call to |
156 | * qed_find_l2_cache_entry(). |
157 | * |
158 | * Called with table_lock held. |
159 | */ |
160 | void qed_commit_l2_cache_entry(L2TableCache *l2_cache, CachedL2Table *l2_table) |
161 | { |
162 | CachedL2Table *entry; |
163 | |
164 | entry = qed_find_l2_cache_entry(l2_cache, l2_table->offset); |
165 | if (entry) { |
166 | qed_unref_l2_cache_entry(entry); |
167 | qed_unref_l2_cache_entry(l2_table); |
168 | return; |
169 | } |
170 | |
171 | /* Evict an unused cache entry so we have space. If all entries are in use |
172 | * we can grow the cache temporarily and we try to shrink back down later. |
173 | */ |
174 | if (l2_cache->n_entries >= MAX_L2_CACHE_SIZE) { |
175 | CachedL2Table *next; |
176 | QTAILQ_FOREACH_SAFE(entry, &l2_cache->entries, node, next) { |
177 | if (entry->ref > 1) { |
178 | continue; |
179 | } |
180 | |
181 | QTAILQ_REMOVE(&l2_cache->entries, entry, node); |
182 | l2_cache->n_entries--; |
183 | qed_unref_l2_cache_entry(entry); |
184 | |
185 | /* Stop evicting when we've shrunk back to max size */ |
186 | if (l2_cache->n_entries < MAX_L2_CACHE_SIZE) { |
187 | break; |
188 | } |
189 | } |
190 | } |
191 | |
192 | l2_cache->n_entries++; |
193 | QTAILQ_INSERT_TAIL(&l2_cache->entries, l2_table, node); |
194 | } |
195 | |