1 // SPDX-License-Identifier: GPL-2.0
5 * Copyright (C) 2013 Red Hat, Inc., Johannes Weiner
8 #include <linux/memcontrol.h>
9 #include <linux/writeback.h>
10 #include <linux/shmem_fs.h>
11 #include <linux/pagemap.h>
12 #include <linux/atomic.h>
13 #include <linux/module.h>
14 #include <linux/swap.h>
15 #include <linux/dax.h>
22 * Per node, two clock lists are maintained for file pages: the
23 * inactive and the active list. Freshly faulted pages start out at
24 * the head of the inactive list and page reclaim scans pages from the
25 * tail. Pages that are accessed multiple times on the inactive list
26 * are promoted to the active list, to protect them from reclaim,
27 * whereas active pages are demoted to the inactive list when the
28 * active list grows too big.
30 * fault ------------------------+
32 * +--------------+ | +-------------+
33 * reclaim <- | inactive | <-+-- demotion | active | <--+
34 * +--------------+ +-------------+ |
36 * +-------------- promotion ------------------+
39 * Access frequency and refault distance
41 * A workload is thrashing when its pages are frequently used but they
42 * are evicted from the inactive list every time before another access
43 * would have promoted them to the active list.
45 * In cases where the average access distance between thrashing pages
46 * is bigger than the size of memory there is nothing that can be
47 * done - the thrashing set could never fit into memory under any
50 * However, the average access distance could be bigger than the
51 * inactive list, yet smaller than the size of memory. In this case,
52 * the set could fit into memory if it weren't for the currently
53 * active pages - which may be used more, hopefully less frequently:
55 * +-memory available to cache-+
57 * +-inactive------+-active----+
58 * a b | c d e f g h i | J K L M N |
59 * +---------------+-----------+
61 * It is prohibitively expensive to accurately track access frequency
62 * of pages. But a reasonable approximation can be made to measure
63 * thrashing on the inactive list, after which refaulting pages can be
64 * activated optimistically to compete with the existing active pages.
66 * Approximating inactive page access frequency - Observations:
68 * 1. When a page is accessed for the first time, it is added to the
69 * head of the inactive list, slides every existing inactive page
70 * towards the tail by one slot, and pushes the current tail page
73 * 2. When a page is accessed for the second time, it is promoted to
74 * the active list, shrinking the inactive list by one slot. This
75 * also slides all inactive pages that were faulted into the cache
76 * more recently than the activated page towards the tail of the
81 * 1. The sum of evictions and activations between any two points in
82 * time indicate the minimum number of inactive pages accessed in
85 * 2. Moving one inactive page N page slots towards the tail of the
86 * list requires at least N inactive page accesses.
90 * 1. When a page is finally evicted from memory, the number of
91 * inactive pages accessed while the page was in cache is at least
92 * the number of page slots on the inactive list.
94 * 2. In addition, measuring the sum of evictions and activations (E)
95 * at the time of a page's eviction, and comparing it to another
96 * reading (R) at the time the page faults back into memory tells
97 * the minimum number of accesses while the page was not cached.
98 * This is called the refault distance.
100 * Because the first access of the page was the fault and the second
101 * access the refault, we combine the in-cache distance with the
102 * out-of-cache distance to get the complete minimum access distance
105 * NR_inactive + (R - E)
107 * And knowing the minimum access distance of a page, we can easily
108 * tell if the page would be able to stay in cache assuming all page
109 * slots in the cache were available:
111 * NR_inactive + (R - E) <= NR_inactive + NR_active
113 * which can be further simplified to
115 * (R - E) <= NR_active
117 * Put into words, the refault distance (out-of-cache) can be seen as
118 * a deficit in inactive list space (in-cache). If the inactive list
119 * had (R - E) more page slots, the page would not have been evicted
120 * in between accesses, but activated instead. And on a full system,
121 * the only thing eating into inactive list space is active pages.
124 * Activating refaulting pages
126 * All that is known about the active list is that the pages have been
127 * accessed more than once in the past. This means that at any given
128 * time there is actually a good chance that pages on the active list
129 * are no longer in active use.
131 * So when a refault distance of (R - E) is observed and there are at
132 * least (R - E) active pages, the refaulting page is activated
133 * optimistically in the hope that (R - E) active pages are actually
134 * used less frequently than the refaulting page - or even not used at
137 * If this is wrong and demotion kicks in, the pages which are truly
138 * used more frequently will be reactivated while the less frequently
139 * used once will be evicted from memory.
141 * But if this is right, the stale pages will be pushed out of memory
142 * and the used pages get to stay in cache.
147 * For each node's file LRU lists, a counter for inactive evictions
148 * and activations is maintained (node->inactive_age).
150 * On eviction, a snapshot of this counter (along with some bits to
151 * identify the node) is stored in the now empty page cache radix tree
152 * slot of the evicted page. This is called a shadow entry.
154 * On cache misses for which there are shadow entries, an eligible
155 * refault distance will immediately activate the refaulting page.
158 #define EVICTION_SHIFT ((BITS_PER_LONG - BITS_PER_XA_VALUE) + \
161 #define EVICTION_MASK (~0UL >> EVICTION_SHIFT)
164 * Eviction timestamps need to be able to cover the full range of
165 * actionable refaults. However, bits are tight in the radix tree
166 * entry, and after storing the identifier for the lruvec there might
167 * not be enough left to represent every single actionable refault. In
168 * that case, we have to sacrifice granularity for distance, and group
169 * evictions into coarser buckets by shaving off lower timestamp bits.
171 static unsigned int bucket_order __read_mostly;
173 static void *pack_shadow(int memcgid, pg_data_t *pgdat, unsigned long eviction)
175 eviction >>= bucket_order;
176 eviction &= EVICTION_MASK;
177 eviction = (eviction << MEM_CGROUP_ID_SHIFT) | memcgid;
178 eviction = (eviction << NODES_SHIFT) | pgdat->node_id;
180 return xa_mk_value(eviction);
183 static void unpack_shadow(void *shadow, int *memcgidp, pg_data_t **pgdat,
184 unsigned long *evictionp)
186 unsigned long entry = xa_to_value(shadow);
189 nid = entry & ((1UL << NODES_SHIFT) - 1);
190 entry >>= NODES_SHIFT;
191 memcgid = entry & ((1UL << MEM_CGROUP_ID_SHIFT) - 1);
192 entry >>= MEM_CGROUP_ID_SHIFT;
195 *pgdat = NODE_DATA(nid);
196 *evictionp = entry << bucket_order;
200 * workingset_eviction - note the eviction of a page from memory
201 * @mapping: address space the page was backing
202 * @page: the page being evicted
204 * Returns a shadow entry to be stored in @mapping->i_pages in place
205 * of the evicted @page so that a later refault can be detected.
207 void *workingset_eviction(struct address_space *mapping, struct page *page)
209 struct mem_cgroup *memcg = page_memcg(page);
210 struct pglist_data *pgdat = page_pgdat(page);
211 int memcgid = mem_cgroup_id(memcg);
212 unsigned long eviction;
213 struct lruvec *lruvec;
215 /* Page is fully exclusive and pins page->mem_cgroup */
216 VM_BUG_ON_PAGE(PageLRU(page), page);
217 VM_BUG_ON_PAGE(page_count(page), page);
218 VM_BUG_ON_PAGE(!PageLocked(page), page);
220 lruvec = mem_cgroup_lruvec(pgdat, memcg);
221 eviction = atomic_long_inc_return(&lruvec->inactive_age);
222 return pack_shadow(memcgid, pgdat, eviction);
226 * workingset_refault - evaluate the refault of a previously evicted page
227 * @shadow: shadow entry of the evicted page
229 * Calculates and evaluates the refault distance of the previously
230 * evicted page in the context of the node it was allocated in.
232 * Returns %true if the page should be activated, %false otherwise.
234 bool workingset_refault(void *shadow)
236 unsigned long refault_distance;
237 unsigned long active_file;
238 struct mem_cgroup *memcg;
239 unsigned long eviction;
240 struct lruvec *lruvec;
241 unsigned long refault;
242 struct pglist_data *pgdat;
245 unpack_shadow(shadow, &memcgid, &pgdat, &eviction);
249 * Look up the memcg associated with the stored ID. It might
250 * have been deleted since the page's eviction.
252 * Note that in rare events the ID could have been recycled
253 * for a new cgroup that refaults a shared page. This is
254 * impossible to tell from the available data. However, this
255 * should be a rare and limited disturbance, and activations
256 * are always speculative anyway. Ultimately, it's the aging
257 * algorithm's job to shake out the minimum access frequency
258 * for the active cache.
260 * XXX: On !CONFIG_MEMCG, this will always return NULL; it
261 * would be better if the root_mem_cgroup existed in all
262 * configurations instead.
264 memcg = mem_cgroup_from_id(memcgid);
265 if (!mem_cgroup_disabled() && !memcg) {
269 lruvec = mem_cgroup_lruvec(pgdat, memcg);
270 refault = atomic_long_read(&lruvec->inactive_age);
271 active_file = lruvec_lru_size(lruvec, LRU_ACTIVE_FILE, MAX_NR_ZONES);
274 * The unsigned subtraction here gives an accurate distance
275 * across inactive_age overflows in most cases.
277 * There is a special case: usually, shadow entries have a
278 * short lifetime and are either refaulted or reclaimed along
279 * with the inode before they get too old. But it is not
280 * impossible for the inactive_age to lap a shadow entry in
281 * the field, which can then can result in a false small
282 * refault distance, leading to a false activation should this
283 * old entry actually refault again. However, earlier kernels
284 * used to deactivate unconditionally with *every* reclaim
285 * invocation for the longest time, so the occasional
286 * inappropriate activation leading to pressure on the active
287 * list is not a problem.
289 refault_distance = (refault - eviction) & EVICTION_MASK;
291 inc_lruvec_state(lruvec, WORKINGSET_REFAULT);
293 if (refault_distance <= active_file) {
294 inc_lruvec_state(lruvec, WORKINGSET_ACTIVATE);
303 * workingset_activation - note a page activation
304 * @page: page that is being activated
306 void workingset_activation(struct page *page)
308 struct mem_cgroup *memcg;
309 struct lruvec *lruvec;
313 * Filter non-memcg pages here, e.g. unmap can call
314 * mark_page_accessed() on VDSO pages.
316 * XXX: See workingset_refault() - this should return
317 * root_mem_cgroup even for !CONFIG_MEMCG.
319 memcg = page_memcg_rcu(page);
320 if (!mem_cgroup_disabled() && !memcg)
322 lruvec = mem_cgroup_lruvec(page_pgdat(page), memcg);
323 atomic_long_inc(&lruvec->inactive_age);
329 * Shadow entries reflect the share of the working set that does not
330 * fit into memory, so their number depends on the access pattern of
331 * the workload. In most cases, they will refault or get reclaimed
332 * along with the inode, but a (malicious) workload that streams
333 * through files with a total size several times that of available
334 * memory, while preventing the inodes from being reclaimed, can
335 * create excessive amounts of shadow nodes. To keep a lid on this,
336 * track shadow nodes and reclaim them when they grow way past the
337 * point where they would still be useful.
340 static struct list_lru shadow_nodes;
342 void workingset_update_node(struct radix_tree_node *node)
345 * Track non-empty nodes that contain only shadow entries;
346 * unlink those that contain pages or are being freed.
348 * Avoid acquiring the list_lru lock when the nodes are
349 * already where they should be. The list_empty() test is safe
350 * as node->private_list is protected by the i_pages lock.
352 if (node->count && node->count == node->exceptional) {
353 if (list_empty(&node->private_list))
354 list_lru_add(&shadow_nodes, &node->private_list);
356 if (!list_empty(&node->private_list))
357 list_lru_del(&shadow_nodes, &node->private_list);
361 static unsigned long count_shadow_nodes(struct shrinker *shrinker,
362 struct shrink_control *sc)
364 unsigned long max_nodes;
368 nodes = list_lru_shrink_count(&shadow_nodes, sc);
371 * Approximate a reasonable limit for the radix tree nodes
372 * containing shadow entries. We don't need to keep more
373 * shadow entries than possible pages on the active list,
374 * since refault distances bigger than that are dismissed.
376 * The size of the active list converges toward 100% of
377 * overall page cache as memory grows, with only a tiny
378 * inactive list. Assume the total cache size for that.
380 * Nodes might be sparsely populated, with only one shadow
381 * entry in the extreme case. Obviously, we cannot keep one
382 * node for every eligible shadow entry, so compromise on a
383 * worst-case density of 1/8th. Below that, not all eligible
384 * refaults can be detected anymore.
386 * On 64-bit with 7 radix_tree_nodes per page and 64 slots
387 * each, this will reclaim shadow entries when they consume
388 * ~1.8% of available memory:
390 * PAGE_SIZE / radix_tree_nodes / node_entries * 8 / PAGE_SIZE
393 cache = mem_cgroup_node_nr_lru_pages(sc->memcg, sc->nid,
396 cache = node_page_state(NODE_DATA(sc->nid), NR_ACTIVE_FILE) +
397 node_page_state(NODE_DATA(sc->nid), NR_INACTIVE_FILE);
399 max_nodes = cache >> (RADIX_TREE_MAP_SHIFT - 3);
404 if (nodes <= max_nodes)
406 return nodes - max_nodes;
409 static enum lru_status shadow_lru_isolate(struct list_head *item,
410 struct list_lru_one *lru,
411 spinlock_t *lru_lock,
414 struct address_space *mapping;
415 struct radix_tree_node *node;
420 * Page cache insertions and deletions synchroneously maintain
421 * the shadow node LRU under the i_pages lock and the
422 * lru_lock. Because the page cache tree is emptied before
423 * the inode can be destroyed, holding the lru_lock pins any
424 * address_space that has radix tree nodes on the LRU.
426 * We can then safely transition to the i_pages lock to
427 * pin only the address_space of the particular node we want
428 * to reclaim, take the node off-LRU, and drop the lru_lock.
431 node = container_of(item, struct radix_tree_node, private_list);
432 mapping = container_of(node->root, struct address_space, i_pages);
434 /* Coming from the list, invert the lock order */
435 if (!xa_trylock(&mapping->i_pages)) {
436 spin_unlock_irq(lru_lock);
441 list_lru_isolate(lru, item);
442 spin_unlock(lru_lock);
445 * The nodes should only contain one or more shadow entries,
446 * no pages, so we expect to be able to remove them all and
447 * delete and free the empty node afterwards.
449 if (WARN_ON_ONCE(!node->exceptional))
451 if (WARN_ON_ONCE(node->count != node->exceptional))
453 for (i = 0; i < RADIX_TREE_MAP_SIZE; i++) {
454 if (node->slots[i]) {
455 if (WARN_ON_ONCE(!xa_is_value(node->slots[i])))
457 if (WARN_ON_ONCE(!node->exceptional))
459 if (WARN_ON_ONCE(!mapping->nrexceptional))
461 node->slots[i] = NULL;
464 mapping->nrexceptional--;
467 if (WARN_ON_ONCE(node->exceptional))
469 inc_lruvec_page_state(virt_to_page(node), WORKINGSET_NODERECLAIM);
470 __radix_tree_delete_node(&mapping->i_pages, node,
471 workingset_lookup_update(mapping));
474 xa_unlock_irq(&mapping->i_pages);
475 ret = LRU_REMOVED_RETRY;
478 spin_lock_irq(lru_lock);
482 static unsigned long scan_shadow_nodes(struct shrinker *shrinker,
483 struct shrink_control *sc)
485 /* list_lru lock nests inside the IRQ-safe i_pages lock */
486 return list_lru_shrink_walk_irq(&shadow_nodes, sc, shadow_lru_isolate,
490 static struct shrinker workingset_shadow_shrinker = {
491 .count_objects = count_shadow_nodes,
492 .scan_objects = scan_shadow_nodes,
493 .seeks = DEFAULT_SEEKS,
494 .flags = SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE,
498 * Our list_lru->lock is IRQ-safe as it nests inside the IRQ-safe
501 static struct lock_class_key shadow_nodes_key;
503 static int __init workingset_init(void)
505 unsigned int timestamp_bits;
506 unsigned int max_order;
509 BUILD_BUG_ON(BITS_PER_LONG < EVICTION_SHIFT);
511 * Calculate the eviction bucket size to cover the longest
512 * actionable refault distance, which is currently half of
513 * memory (totalram_pages/2). However, memory hotplug may add
514 * some more pages at runtime, so keep working with up to
515 * double the initial memory by using totalram_pages as-is.
517 timestamp_bits = BITS_PER_LONG - EVICTION_SHIFT;
518 max_order = fls_long(totalram_pages - 1);
519 if (max_order > timestamp_bits)
520 bucket_order = max_order - timestamp_bits;
521 pr_info("workingset: timestamp_bits=%d max_order=%d bucket_order=%u\n",
522 timestamp_bits, max_order, bucket_order);
524 ret = prealloc_shrinker(&workingset_shadow_shrinker);
527 ret = __list_lru_init(&shadow_nodes, true, &shadow_nodes_key,
528 &workingset_shadow_shrinker);
531 register_shrinker_prepared(&workingset_shadow_shrinker);
534 free_prealloced_shrinker(&workingset_shadow_shrinker);
538 module_init(workingset_init);