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1/*
2 * Workingset detection
3 *
4 * Copyright (C) 2013 Red Hat, Inc., Johannes Weiner
5 */
6
7#include <linux/memcontrol.h>
8#include <linux/writeback.h>
9#include <linux/pagemap.h>
10#include <linux/atomic.h>
11#include <linux/module.h>
12#include <linux/swap.h>
13#include <linux/dax.h>
14#include <linux/fs.h>
15#include <linux/mm.h>
16
17/*
18 * Double CLOCK lists
19 *
20 * Per node, two clock lists are maintained for file pages: the
21 * inactive and the active list. Freshly faulted pages start out at
22 * the head of the inactive list and page reclaim scans pages from the
23 * tail. Pages that are accessed multiple times on the inactive list
24 * are promoted to the active list, to protect them from reclaim,
25 * whereas active pages are demoted to the inactive list when the
26 * active list grows too big.
27 *
28 * fault ------------------------+
29 * |
30 * +--------------+ | +-------------+
31 * reclaim <- | inactive | <-+-- demotion | active | <--+
32 * +--------------+ +-------------+ |
33 * | |
34 * +-------------- promotion ------------------+
35 *
36 *
37 * Access frequency and refault distance
38 *
39 * A workload is thrashing when its pages are frequently used but they
40 * are evicted from the inactive list every time before another access
41 * would have promoted them to the active list.
42 *
43 * In cases where the average access distance between thrashing pages
44 * is bigger than the size of memory there is nothing that can be
45 * done - the thrashing set could never fit into memory under any
46 * circumstance.
47 *
48 * However, the average access distance could be bigger than the
49 * inactive list, yet smaller than the size of memory. In this case,
50 * the set could fit into memory if it weren't for the currently
51 * active pages - which may be used more, hopefully less frequently:
52 *
53 * +-memory available to cache-+
54 * | |
55 * +-inactive------+-active----+
56 * a b | c d e f g h i | J K L M N |
57 * +---------------+-----------+
58 *
59 * It is prohibitively expensive to accurately track access frequency
60 * of pages. But a reasonable approximation can be made to measure
61 * thrashing on the inactive list, after which refaulting pages can be
62 * activated optimistically to compete with the existing active pages.
63 *
64 * Approximating inactive page access frequency - Observations:
65 *
66 * 1. When a page is accessed for the first time, it is added to the
67 * head of the inactive list, slides every existing inactive page
68 * towards the tail by one slot, and pushes the current tail page
69 * out of memory.
70 *
71 * 2. When a page is accessed for the second time, it is promoted to
72 * the active list, shrinking the inactive list by one slot. This
73 * also slides all inactive pages that were faulted into the cache
74 * more recently than the activated page towards the tail of the
75 * inactive list.
76 *
77 * Thus:
78 *
79 * 1. The sum of evictions and activations between any two points in
80 * time indicate the minimum number of inactive pages accessed in
81 * between.
82 *
83 * 2. Moving one inactive page N page slots towards the tail of the
84 * list requires at least N inactive page accesses.
85 *
86 * Combining these:
87 *
88 * 1. When a page is finally evicted from memory, the number of
89 * inactive pages accessed while the page was in cache is at least
90 * the number of page slots on the inactive list.
91 *
92 * 2. In addition, measuring the sum of evictions and activations (E)
93 * at the time of a page's eviction, and comparing it to another
94 * reading (R) at the time the page faults back into memory tells
95 * the minimum number of accesses while the page was not cached.
96 * This is called the refault distance.
97 *
98 * Because the first access of the page was the fault and the second
99 * access the refault, we combine the in-cache distance with the
100 * out-of-cache distance to get the complete minimum access distance
101 * of this page:
102 *
103 * NR_inactive + (R - E)
104 *
105 * And knowing the minimum access distance of a page, we can easily
106 * tell if the page would be able to stay in cache assuming all page
107 * slots in the cache were available:
108 *
109 * NR_inactive + (R - E) <= NR_inactive + NR_active
110 *
111 * which can be further simplified to
112 *
113 * (R - E) <= NR_active
114 *
115 * Put into words, the refault distance (out-of-cache) can be seen as
116 * a deficit in inactive list space (in-cache). If the inactive list
117 * had (R - E) more page slots, the page would not have been evicted
118 * in between accesses, but activated instead. And on a full system,
119 * the only thing eating into inactive list space is active pages.
120 *
121 *
122 * Activating refaulting pages
123 *
124 * All that is known about the active list is that the pages have been
125 * accessed more than once in the past. This means that at any given
126 * time there is actually a good chance that pages on the active list
127 * are no longer in active use.
128 *
129 * So when a refault distance of (R - E) is observed and there are at
130 * least (R - E) active pages, the refaulting page is activated
131 * optimistically in the hope that (R - E) active pages are actually
132 * used less frequently than the refaulting page - or even not used at
133 * all anymore.
134 *
135 * If this is wrong and demotion kicks in, the pages which are truly
136 * used more frequently will be reactivated while the less frequently
137 * used once will be evicted from memory.
138 *
139 * But if this is right, the stale pages will be pushed out of memory
140 * and the used pages get to stay in cache.
141 *
142 *
143 * Implementation
144 *
145 * For each node's file LRU lists, a counter for inactive evictions
146 * and activations is maintained (node->inactive_age).
147 *
148 * On eviction, a snapshot of this counter (along with some bits to
149 * identify the node) is stored in the now empty page cache radix tree
150 * slot of the evicted page. This is called a shadow entry.
151 *
152 * On cache misses for which there are shadow entries, an eligible
153 * refault distance will immediately activate the refaulting page.
154 */
155
156#define EVICTION_SHIFT (RADIX_TREE_EXCEPTIONAL_ENTRY + \
157 NODES_SHIFT + \
158 MEM_CGROUP_ID_SHIFT)
159#define EVICTION_MASK (~0UL >> EVICTION_SHIFT)
160
161/*
162 * Eviction timestamps need to be able to cover the full range of
163 * actionable refaults. However, bits are tight in the radix tree
164 * entry, and after storing the identifier for the lruvec there might
165 * not be enough left to represent every single actionable refault. In
166 * that case, we have to sacrifice granularity for distance, and group
167 * evictions into coarser buckets by shaving off lower timestamp bits.
168 */
169static unsigned int bucket_order __read_mostly;
170
171static void *pack_shadow(int memcgid, pg_data_t *pgdat, unsigned long eviction)
172{
173 eviction >>= bucket_order;
174 eviction = (eviction << MEM_CGROUP_ID_SHIFT) | memcgid;
175 eviction = (eviction << NODES_SHIFT) | pgdat->node_id;
176 eviction = (eviction << RADIX_TREE_EXCEPTIONAL_SHIFT);
177
178 return (void *)(eviction | RADIX_TREE_EXCEPTIONAL_ENTRY);
179}
180
181static void unpack_shadow(void *shadow, int *memcgidp, pg_data_t **pgdat,
182 unsigned long *evictionp)
183{
184 unsigned long entry = (unsigned long)shadow;
185 int memcgid, nid;
186
187 entry >>= RADIX_TREE_EXCEPTIONAL_SHIFT;
188 nid = entry & ((1UL << NODES_SHIFT) - 1);
189 entry >>= NODES_SHIFT;
190 memcgid = entry & ((1UL << MEM_CGROUP_ID_SHIFT) - 1);
191 entry >>= MEM_CGROUP_ID_SHIFT;
192
193 *memcgidp = memcgid;
194 *pgdat = NODE_DATA(nid);
195 *evictionp = entry << bucket_order;
196}
197
198/**
199 * workingset_eviction - note the eviction of a page from memory
200 * @mapping: address space the page was backing
201 * @page: the page being evicted
202 *
203 * Returns a shadow entry to be stored in @mapping->page_tree in place
204 * of the evicted @page so that a later refault can be detected.
205 */
206void *workingset_eviction(struct address_space *mapping, struct page *page)
207{
208 struct mem_cgroup *memcg = page_memcg(page);
209 struct pglist_data *pgdat = page_pgdat(page);
210 int memcgid = mem_cgroup_id(memcg);
211 unsigned long eviction;
212 struct lruvec *lruvec;
213
214 /* Page is fully exclusive and pins page->mem_cgroup */
215 VM_BUG_ON_PAGE(PageLRU(page), page);
216 VM_BUG_ON_PAGE(page_count(page), page);
217 VM_BUG_ON_PAGE(!PageLocked(page), page);
218
219 lruvec = mem_cgroup_lruvec(pgdat, memcg);
220 eviction = atomic_long_inc_return(&lruvec->inactive_age);
221 return pack_shadow(memcgid, pgdat, eviction);
222}
223
224/**
225 * workingset_refault - evaluate the refault of a previously evicted page
226 * @shadow: shadow entry of the evicted page
227 *
228 * Calculates and evaluates the refault distance of the previously
229 * evicted page in the context of the node it was allocated in.
230 *
231 * Returns %true if the page should be activated, %false otherwise.
232 */
233bool workingset_refault(void *shadow)
234{
235 unsigned long refault_distance;
236 unsigned long active_file;
237 struct mem_cgroup *memcg;
238 unsigned long eviction;
239 struct lruvec *lruvec;
240 unsigned long refault;
241 struct pglist_data *pgdat;
242 int memcgid;
243
244 unpack_shadow(shadow, &memcgid, &pgdat, &eviction);
245
246 rcu_read_lock();
247 /*
248 * Look up the memcg associated with the stored ID. It might
249 * have been deleted since the page's eviction.
250 *
251 * Note that in rare events the ID could have been recycled
252 * for a new cgroup that refaults a shared page. This is
253 * impossible to tell from the available data. However, this
254 * should be a rare and limited disturbance, and activations
255 * are always speculative anyway. Ultimately, it's the aging
256 * algorithm's job to shake out the minimum access frequency
257 * for the active cache.
258 *
259 * XXX: On !CONFIG_MEMCG, this will always return NULL; it
260 * would be better if the root_mem_cgroup existed in all
261 * configurations instead.
262 */
263 memcg = mem_cgroup_from_id(memcgid);
264 if (!mem_cgroup_disabled() && !memcg) {
265 rcu_read_unlock();
266 return false;
267 }
268 lruvec = mem_cgroup_lruvec(pgdat, memcg);
269 refault = atomic_long_read(&lruvec->inactive_age);
270 active_file = lruvec_lru_size(lruvec, LRU_ACTIVE_FILE, MAX_NR_ZONES);
271 rcu_read_unlock();
272
273 /*
274 * The unsigned subtraction here gives an accurate distance
275 * across inactive_age overflows in most cases.
276 *
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.
288 */
289 refault_distance = (refault - eviction) & EVICTION_MASK;
290
291 inc_node_state(pgdat, WORKINGSET_REFAULT);
292
293 if (refault_distance <= active_file) {
294 inc_node_state(pgdat, WORKINGSET_ACTIVATE);
295 return true;
296 }
297 return false;
298}
299
300/**
301 * workingset_activation - note a page activation
302 * @page: page that is being activated
303 */
304void workingset_activation(struct page *page)
305{
306 struct mem_cgroup *memcg;
307 struct lruvec *lruvec;
308
309 rcu_read_lock();
310 /*
311 * Filter non-memcg pages here, e.g. unmap can call
312 * mark_page_accessed() on VDSO pages.
313 *
314 * XXX: See workingset_refault() - this should return
315 * root_mem_cgroup even for !CONFIG_MEMCG.
316 */
317 memcg = page_memcg_rcu(page);
318 if (!mem_cgroup_disabled() && !memcg)
319 goto out;
320 lruvec = mem_cgroup_lruvec(page_pgdat(page), memcg);
321 atomic_long_inc(&lruvec->inactive_age);
322out:
323 rcu_read_unlock();
324}
325
326/*
327 * Shadow entries reflect the share of the working set that does not
328 * fit into memory, so their number depends on the access pattern of
329 * the workload. In most cases, they will refault or get reclaimed
330 * along with the inode, but a (malicious) workload that streams
331 * through files with a total size several times that of available
332 * memory, while preventing the inodes from being reclaimed, can
333 * create excessive amounts of shadow nodes. To keep a lid on this,
334 * track shadow nodes and reclaim them when they grow way past the
335 * point where they would still be useful.
336 */
337
338static struct list_lru shadow_nodes;
339
340void workingset_update_node(struct radix_tree_node *node, void *private)
341{
342 struct address_space *mapping = private;
343
344 /* Only regular page cache has shadow entries */
345 if (dax_mapping(mapping) || shmem_mapping(mapping))
346 return;
347
348 /*
349 * Track non-empty nodes that contain only shadow entries;
350 * unlink those that contain pages or are being freed.
351 *
352 * Avoid acquiring the list_lru lock when the nodes are
353 * already where they should be. The list_empty() test is safe
354 * as node->private_list is protected by &mapping->tree_lock.
355 */
356 if (node->count && node->count == node->exceptional) {
357 if (list_empty(&node->private_list)) {
358 node->private_data = mapping;
359 list_lru_add(&shadow_nodes, &node->private_list);
360 }
361 } else {
362 if (!list_empty(&node->private_list))
363 list_lru_del(&shadow_nodes, &node->private_list);
364 }
365}
366
367static unsigned long count_shadow_nodes(struct shrinker *shrinker,
368 struct shrink_control *sc)
369{
370 unsigned long max_nodes;
371 unsigned long nodes;
372 unsigned long cache;
373
374 /* list_lru lock nests inside IRQ-safe mapping->tree_lock */
375 local_irq_disable();
376 nodes = list_lru_shrink_count(&shadow_nodes, sc);
377 local_irq_enable();
378
379 /*
380 * Approximate a reasonable limit for the radix tree nodes
381 * containing shadow entries. We don't need to keep more
382 * shadow entries than possible pages on the active list,
383 * since refault distances bigger than that are dismissed.
384 *
385 * The size of the active list converges toward 100% of
386 * overall page cache as memory grows, with only a tiny
387 * inactive list. Assume the total cache size for that.
388 *
389 * Nodes might be sparsely populated, with only one shadow
390 * entry in the extreme case. Obviously, we cannot keep one
391 * node for every eligible shadow entry, so compromise on a
392 * worst-case density of 1/8th. Below that, not all eligible
393 * refaults can be detected anymore.
394 *
395 * On 64-bit with 7 radix_tree_nodes per page and 64 slots
396 * each, this will reclaim shadow entries when they consume
397 * ~1.8% of available memory:
398 *
399 * PAGE_SIZE / radix_tree_nodes / node_entries * 8 / PAGE_SIZE
400 */
401 if (sc->memcg) {
402 cache = mem_cgroup_node_nr_lru_pages(sc->memcg, sc->nid,
403 LRU_ALL_FILE);
404 } else {
405 cache = node_page_state(NODE_DATA(sc->nid), NR_ACTIVE_FILE) +
406 node_page_state(NODE_DATA(sc->nid), NR_INACTIVE_FILE);
407 }
408 max_nodes = cache >> (RADIX_TREE_MAP_SHIFT - 3);
409
410 if (nodes <= max_nodes)
411 return 0;
412 return nodes - max_nodes;
413}
414
415static enum lru_status shadow_lru_isolate(struct list_head *item,
416 struct list_lru_one *lru,
417 spinlock_t *lru_lock,
418 void *arg)
419{
420 struct address_space *mapping;
421 struct radix_tree_node *node;
422 unsigned int i;
423 int ret;
424
425 /*
426 * Page cache insertions and deletions synchroneously maintain
427 * the shadow node LRU under the mapping->tree_lock and the
428 * lru_lock. Because the page cache tree is emptied before
429 * the inode can be destroyed, holding the lru_lock pins any
430 * address_space that has radix tree nodes on the LRU.
431 *
432 * We can then safely transition to the mapping->tree_lock to
433 * pin only the address_space of the particular node we want
434 * to reclaim, take the node off-LRU, and drop the lru_lock.
435 */
436
437 node = container_of(item, struct radix_tree_node, private_list);
438 mapping = node->private_data;
439
440 /* Coming from the list, invert the lock order */
441 if (!spin_trylock(&mapping->tree_lock)) {
442 spin_unlock(lru_lock);
443 ret = LRU_RETRY;
444 goto out;
445 }
446
447 list_lru_isolate(lru, item);
448 spin_unlock(lru_lock);
449
450 /*
451 * The nodes should only contain one or more shadow entries,
452 * no pages, so we expect to be able to remove them all and
453 * delete and free the empty node afterwards.
454 */
455 if (WARN_ON_ONCE(!node->exceptional))
456 goto out_invalid;
457 if (WARN_ON_ONCE(node->count != node->exceptional))
458 goto out_invalid;
459 for (i = 0; i < RADIX_TREE_MAP_SIZE; i++) {
460 if (node->slots[i]) {
461 if (WARN_ON_ONCE(!radix_tree_exceptional_entry(node->slots[i])))
462 goto out_invalid;
463 if (WARN_ON_ONCE(!node->exceptional))
464 goto out_invalid;
465 if (WARN_ON_ONCE(!mapping->nrexceptional))
466 goto out_invalid;
467 node->slots[i] = NULL;
468 node->exceptional--;
469 node->count--;
470 mapping->nrexceptional--;
471 }
472 }
473 if (WARN_ON_ONCE(node->exceptional))
474 goto out_invalid;
475 inc_node_state(page_pgdat(virt_to_page(node)), WORKINGSET_NODERECLAIM);
476 __radix_tree_delete_node(&mapping->page_tree, node,
477 workingset_update_node, mapping);
478
479out_invalid:
480 spin_unlock(&mapping->tree_lock);
481 ret = LRU_REMOVED_RETRY;
482out:
483 local_irq_enable();
484 cond_resched();
485 local_irq_disable();
486 spin_lock(lru_lock);
487 return ret;
488}
489
490static unsigned long scan_shadow_nodes(struct shrinker *shrinker,
491 struct shrink_control *sc)
492{
493 unsigned long ret;
494
495 /* list_lru lock nests inside IRQ-safe mapping->tree_lock */
496 local_irq_disable();
497 ret = list_lru_shrink_walk(&shadow_nodes, sc, shadow_lru_isolate, NULL);
498 local_irq_enable();
499 return ret;
500}
501
502static struct shrinker workingset_shadow_shrinker = {
503 .count_objects = count_shadow_nodes,
504 .scan_objects = scan_shadow_nodes,
505 .seeks = DEFAULT_SEEKS,
506 .flags = SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE,
507};
508
509/*
510 * Our list_lru->lock is IRQ-safe as it nests inside the IRQ-safe
511 * mapping->tree_lock.
512 */
513static struct lock_class_key shadow_nodes_key;
514
515static int __init workingset_init(void)
516{
517 unsigned int timestamp_bits;
518 unsigned int max_order;
519 int ret;
520
521 BUILD_BUG_ON(BITS_PER_LONG < EVICTION_SHIFT);
522 /*
523 * Calculate the eviction bucket size to cover the longest
524 * actionable refault distance, which is currently half of
525 * memory (totalram_pages/2). However, memory hotplug may add
526 * some more pages at runtime, so keep working with up to
527 * double the initial memory by using totalram_pages as-is.
528 */
529 timestamp_bits = BITS_PER_LONG - EVICTION_SHIFT;
530 max_order = fls_long(totalram_pages - 1);
531 if (max_order > timestamp_bits)
532 bucket_order = max_order - timestamp_bits;
533 pr_info("workingset: timestamp_bits=%d max_order=%d bucket_order=%u\n",
534 timestamp_bits, max_order, bucket_order);
535
536 ret = __list_lru_init(&shadow_nodes, true, &shadow_nodes_key);
537 if (ret)
538 goto err;
539 ret = register_shrinker(&workingset_shadow_shrinker);
540 if (ret)
541 goto err_list_lru;
542 return 0;
543err_list_lru:
544 list_lru_destroy(&shadow_nodes);
545err:
546 return ret;
547}
548module_init(workingset_init);