1 // SPDX-License-Identifier: GPL-2.0
2 /*
3 * Workingset detection
4 *
5 * Copyright (C) 2013 Red Hat, Inc., Johannes Weiner
6 */
7
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>
16 #include <linux/fs.h>
17 #include <linux/mm.h>
18
19 /*
20 * Double CLOCK lists
21 *
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.
29 *
30 * fault ------------------------+
31 * |
32 * +--------------+ | +-------------+
33 * reclaim <- | inactive | <-+-- demotion | active | <--+
34 * +--------------+ +-------------+ |
35 * | |
36 * +-------------- promotion ------------------+
37 *
38 *
39 * Access frequency and refault distance
40 *
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.
44 *
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
48 * circumstance.
49 *
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:
54 *
55 * +-memory available to cache-+
56 * | |
57 * +-inactive------+-active----+
58 * a b | c d e f g h i | J K L M N |
59 * +---------------+-----------+
60 *
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.
65 *
66 * Approximating inactive page access frequency - Observations:
67 *
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
71 * out of memory.
72 *
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
77 * inactive list.
78 *
79 * Thus:
80 *
81 * 1. The sum of evictions and activations between any two points in
82 * time indicate the minimum number of inactive pages accessed in
83 * between.
84 *
85 * 2. Moving one inactive page N page slots towards the tail of the
86 * list requires at least N inactive page accesses.
87 *
88 * Combining these:
89 *
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.
93 *
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.
99 *
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
103 * of this page:
104 *
105 * NR_inactive + (R - E)
106 *
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:
110 *
111 * NR_inactive + (R - E) <= NR_inactive + NR_active
112 *
113 * which can be further simplified to
114 *
115 * (R - E) <= NR_active
116 *
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.
122 *
123 *
124 * Activating refaulting pages
125 *
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.
130 *
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
135 * all anymore.
136 *
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.
140 *
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.
143 *
144 *
145 * Implementation
146 *
147 * For each node's file LRU lists, a counter for inactive evictions
148 * and activations is maintained (node->inactive_age).
149 *
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.
153 *
154 * On cache misses for which there are shadow entries, an eligible
155 * refault distance will immediately activate the refaulting page.
156 */
157
158 #define EVICTION_SHIFT (RADIX_TREE_EXCEPTIONAL_ENTRY + \
159 NODES_SHIFT + \
160 MEM_CGROUP_ID_SHIFT)
161 #define EVICTION_MASK (~0UL >> EVICTION_SHIFT)
162
163 /*
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.
170 */
171 static unsigned int bucket_order __read_mostly;
172
pack_shadow(int memcgid,pg_data_t * pgdat,unsigned long eviction)173 static void *pack_shadow(int memcgid, pg_data_t *pgdat, unsigned long eviction)
174 {
175 eviction >>= bucket_order;
176 eviction = (eviction << MEM_CGROUP_ID_SHIFT) | memcgid;
177 eviction = (eviction << NODES_SHIFT) | pgdat->node_id;
178 eviction = (eviction << RADIX_TREE_EXCEPTIONAL_SHIFT);
179
180 return (void *)(eviction | RADIX_TREE_EXCEPTIONAL_ENTRY);
181 }
182
unpack_shadow(void * shadow,int * memcgidp,pg_data_t ** pgdat,unsigned long * evictionp)183 static void unpack_shadow(void *shadow, int *memcgidp, pg_data_t **pgdat,
184 unsigned long *evictionp)
185 {
186 unsigned long entry = (unsigned long)shadow;
187 int memcgid, nid;
188
189 entry >>= RADIX_TREE_EXCEPTIONAL_SHIFT;
190 nid = entry & ((1UL << NODES_SHIFT) - 1);
191 entry >>= NODES_SHIFT;
192 memcgid = entry & ((1UL << MEM_CGROUP_ID_SHIFT) - 1);
193 entry >>= MEM_CGROUP_ID_SHIFT;
194
195 *memcgidp = memcgid;
196 *pgdat = NODE_DATA(nid);
197 *evictionp = entry << bucket_order;
198 }
199
200 /**
201 * workingset_eviction - note the eviction of a page from memory
202 * @mapping: address space the page was backing
203 * @page: the page being evicted
204 *
205 * Returns a shadow entry to be stored in @mapping->i_pages in place
206 * of the evicted @page so that a later refault can be detected.
207 */
workingset_eviction(struct address_space * mapping,struct page * page)208 void *workingset_eviction(struct address_space *mapping, struct page *page)
209 {
210 struct mem_cgroup *memcg = page_memcg(page);
211 struct pglist_data *pgdat = page_pgdat(page);
212 int memcgid = mem_cgroup_id(memcg);
213 unsigned long eviction;
214 struct lruvec *lruvec;
215
216 /* Page is fully exclusive and pins page->mem_cgroup */
217 VM_BUG_ON_PAGE(PageLRU(page), page);
218 VM_BUG_ON_PAGE(page_count(page), page);
219 VM_BUG_ON_PAGE(!PageLocked(page), page);
220
221 lruvec = mem_cgroup_lruvec(pgdat, memcg);
222 eviction = atomic_long_inc_return(&lruvec->inactive_age);
223 return pack_shadow(memcgid, pgdat, eviction);
224 }
225
226 /**
227 * workingset_refault - evaluate the refault of a previously evicted page
228 * @shadow: shadow entry of the evicted page
229 *
230 * Calculates and evaluates the refault distance of the previously
231 * evicted page in the context of the node it was allocated in.
232 *
233 * Returns %true if the page should be activated, %false otherwise.
234 */
workingset_refault(void * shadow)235 bool workingset_refault(void *shadow)
236 {
237 unsigned long refault_distance;
238 unsigned long active_file;
239 struct mem_cgroup *memcg;
240 unsigned long eviction;
241 struct lruvec *lruvec;
242 unsigned long refault;
243 struct pglist_data *pgdat;
244 int memcgid;
245
246 unpack_shadow(shadow, &memcgid, &pgdat, &eviction);
247
248 rcu_read_lock();
249 /*
250 * Look up the memcg associated with the stored ID. It might
251 * have been deleted since the page's eviction.
252 *
253 * Note that in rare events the ID could have been recycled
254 * for a new cgroup that refaults a shared page. This is
255 * impossible to tell from the available data. However, this
256 * should be a rare and limited disturbance, and activations
257 * are always speculative anyway. Ultimately, it's the aging
258 * algorithm's job to shake out the minimum access frequency
259 * for the active cache.
260 *
261 * XXX: On !CONFIG_MEMCG, this will always return NULL; it
262 * would be better if the root_mem_cgroup existed in all
263 * configurations instead.
264 */
265 memcg = mem_cgroup_from_id(memcgid);
266 if (!mem_cgroup_disabled() && !memcg) {
267 rcu_read_unlock();
268 return false;
269 }
270 lruvec = mem_cgroup_lruvec(pgdat, memcg);
271 refault = atomic_long_read(&lruvec->inactive_age);
272 active_file = lruvec_lru_size(lruvec, LRU_ACTIVE_FILE, MAX_NR_ZONES);
273
274 /*
275 * The unsigned subtraction here gives an accurate distance
276 * across inactive_age overflows in most cases.
277 *
278 * There is a special case: usually, shadow entries have a
279 * short lifetime and are either refaulted or reclaimed along
280 * with the inode before they get too old. But it is not
281 * impossible for the inactive_age to lap a shadow entry in
282 * the field, which can then can result in a false small
283 * refault distance, leading to a false activation should this
284 * old entry actually refault again. However, earlier kernels
285 * used to deactivate unconditionally with *every* reclaim
286 * invocation for the longest time, so the occasional
287 * inappropriate activation leading to pressure on the active
288 * list is not a problem.
289 */
290 refault_distance = (refault - eviction) & EVICTION_MASK;
291
292 inc_lruvec_state(lruvec, WORKINGSET_REFAULT);
293
294 if (refault_distance <= active_file) {
295 inc_lruvec_state(lruvec, WORKINGSET_ACTIVATE);
296 rcu_read_unlock();
297 return true;
298 }
299 rcu_read_unlock();
300 return false;
301 }
302
303 /**
304 * workingset_activation - note a page activation
305 * @page: page that is being activated
306 */
workingset_activation(struct page * page)307 void workingset_activation(struct page *page)
308 {
309 struct mem_cgroup *memcg;
310 struct lruvec *lruvec;
311
312 rcu_read_lock();
313 /*
314 * Filter non-memcg pages here, e.g. unmap can call
315 * mark_page_accessed() on VDSO pages.
316 *
317 * XXX: See workingset_refault() - this should return
318 * root_mem_cgroup even for !CONFIG_MEMCG.
319 */
320 memcg = page_memcg_rcu(page);
321 if (!mem_cgroup_disabled() && !memcg)
322 goto out;
323 lruvec = mem_cgroup_lruvec(page_pgdat(page), memcg);
324 atomic_long_inc(&lruvec->inactive_age);
325 out:
326 rcu_read_unlock();
327 }
328
329 /*
330 * Shadow entries reflect the share of the working set that does not
331 * fit into memory, so their number depends on the access pattern of
332 * the workload. In most cases, they will refault or get reclaimed
333 * along with the inode, but a (malicious) workload that streams
334 * through files with a total size several times that of available
335 * memory, while preventing the inodes from being reclaimed, can
336 * create excessive amounts of shadow nodes. To keep a lid on this,
337 * track shadow nodes and reclaim them when they grow way past the
338 * point where they would still be useful.
339 */
340
341 static struct list_lru shadow_nodes;
342
workingset_update_node(struct radix_tree_node * node)343 void workingset_update_node(struct radix_tree_node *node)
344 {
345 /*
346 * Track non-empty nodes that contain only shadow entries;
347 * unlink those that contain pages or are being freed.
348 *
349 * Avoid acquiring the list_lru lock when the nodes are
350 * already where they should be. The list_empty() test is safe
351 * as node->private_list is protected by the i_pages lock.
352 */
353 if (node->count && node->count == node->exceptional) {
354 if (list_empty(&node->private_list))
355 list_lru_add(&shadow_nodes, &node->private_list);
356 } else {
357 if (!list_empty(&node->private_list))
358 list_lru_del(&shadow_nodes, &node->private_list);
359 }
360 }
361
count_shadow_nodes(struct shrinker * shrinker,struct shrink_control * sc)362 static unsigned long count_shadow_nodes(struct shrinker *shrinker,
363 struct shrink_control *sc)
364 {
365 unsigned long max_nodes;
366 unsigned long nodes;
367 unsigned long cache;
368
369 nodes = list_lru_shrink_count(&shadow_nodes, sc);
370
371 /*
372 * Approximate a reasonable limit for the radix tree nodes
373 * containing shadow entries. We don't need to keep more
374 * shadow entries than possible pages on the active list,
375 * since refault distances bigger than that are dismissed.
376 *
377 * The size of the active list converges toward 100% of
378 * overall page cache as memory grows, with only a tiny
379 * inactive list. Assume the total cache size for that.
380 *
381 * Nodes might be sparsely populated, with only one shadow
382 * entry in the extreme case. Obviously, we cannot keep one
383 * node for every eligible shadow entry, so compromise on a
384 * worst-case density of 1/8th. Below that, not all eligible
385 * refaults can be detected anymore.
386 *
387 * On 64-bit with 7 radix_tree_nodes per page and 64 slots
388 * each, this will reclaim shadow entries when they consume
389 * ~1.8% of available memory:
390 *
391 * PAGE_SIZE / radix_tree_nodes / node_entries * 8 / PAGE_SIZE
392 */
393 if (sc->memcg) {
394 cache = mem_cgroup_node_nr_lru_pages(sc->memcg, sc->nid,
395 LRU_ALL_FILE);
396 } else {
397 cache = node_page_state(NODE_DATA(sc->nid), NR_ACTIVE_FILE) +
398 node_page_state(NODE_DATA(sc->nid), NR_INACTIVE_FILE);
399 }
400 max_nodes = cache >> (RADIX_TREE_MAP_SHIFT - 3);
401
402 if (!nodes)
403 return SHRINK_EMPTY;
404
405 if (nodes <= max_nodes)
406 return 0;
407 return nodes - max_nodes;
408 }
409
shadow_lru_isolate(struct list_head * item,struct list_lru_one * lru,spinlock_t * lru_lock,void * arg)410 static enum lru_status shadow_lru_isolate(struct list_head *item,
411 struct list_lru_one *lru,
412 spinlock_t *lru_lock,
413 void *arg)
414 {
415 struct address_space *mapping;
416 struct radix_tree_node *node;
417 unsigned int i;
418 int ret;
419
420 /*
421 * Page cache insertions and deletions synchroneously maintain
422 * the shadow node LRU under the i_pages lock and the
423 * lru_lock. Because the page cache tree is emptied before
424 * the inode can be destroyed, holding the lru_lock pins any
425 * address_space that has radix tree nodes on the LRU.
426 *
427 * We can then safely transition to the i_pages lock to
428 * pin only the address_space of the particular node we want
429 * to reclaim, take the node off-LRU, and drop the lru_lock.
430 */
431
432 node = container_of(item, struct radix_tree_node, private_list);
433 mapping = container_of(node->root, struct address_space, i_pages);
434
435 /* Coming from the list, invert the lock order */
436 if (!xa_trylock(&mapping->i_pages)) {
437 spin_unlock_irq(lru_lock);
438 ret = LRU_RETRY;
439 goto out;
440 }
441
442 list_lru_isolate(lru, item);
443 spin_unlock(lru_lock);
444
445 /*
446 * The nodes should only contain one or more shadow entries,
447 * no pages, so we expect to be able to remove them all and
448 * delete and free the empty node afterwards.
449 */
450 if (WARN_ON_ONCE(!node->exceptional))
451 goto out_invalid;
452 if (WARN_ON_ONCE(node->count != node->exceptional))
453 goto out_invalid;
454 for (i = 0; i < RADIX_TREE_MAP_SIZE; i++) {
455 if (node->slots[i]) {
456 if (WARN_ON_ONCE(!radix_tree_exceptional_entry(node->slots[i])))
457 goto out_invalid;
458 if (WARN_ON_ONCE(!node->exceptional))
459 goto out_invalid;
460 if (WARN_ON_ONCE(!mapping->nrexceptional))
461 goto out_invalid;
462 node->slots[i] = NULL;
463 node->exceptional--;
464 node->count--;
465 mapping->nrexceptional--;
466 }
467 }
468 if (WARN_ON_ONCE(node->exceptional))
469 goto out_invalid;
470 inc_lruvec_page_state(virt_to_page(node), WORKINGSET_NODERECLAIM);
471 __radix_tree_delete_node(&mapping->i_pages, node,
472 workingset_lookup_update(mapping));
473
474 out_invalid:
475 xa_unlock_irq(&mapping->i_pages);
476 ret = LRU_REMOVED_RETRY;
477 out:
478 cond_resched();
479 spin_lock_irq(lru_lock);
480 return ret;
481 }
482
scan_shadow_nodes(struct shrinker * shrinker,struct shrink_control * sc)483 static unsigned long scan_shadow_nodes(struct shrinker *shrinker,
484 struct shrink_control *sc)
485 {
486 /* list_lru lock nests inside the IRQ-safe i_pages lock */
487 return list_lru_shrink_walk_irq(&shadow_nodes, sc, shadow_lru_isolate,
488 NULL);
489 }
490
491 static struct shrinker workingset_shadow_shrinker = {
492 .count_objects = count_shadow_nodes,
493 .scan_objects = scan_shadow_nodes,
494 .seeks = DEFAULT_SEEKS,
495 .flags = SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE,
496 };
497
498 /*
499 * Our list_lru->lock is IRQ-safe as it nests inside the IRQ-safe
500 * i_pages lock.
501 */
502 static struct lock_class_key shadow_nodes_key;
503
workingset_init(void)504 static int __init workingset_init(void)
505 {
506 unsigned int timestamp_bits;
507 unsigned int max_order;
508 int ret;
509
510 BUILD_BUG_ON(BITS_PER_LONG < EVICTION_SHIFT);
511 /*
512 * Calculate the eviction bucket size to cover the longest
513 * actionable refault distance, which is currently half of
514 * memory (totalram_pages/2). However, memory hotplug may add
515 * some more pages at runtime, so keep working with up to
516 * double the initial memory by using totalram_pages as-is.
517 */
518 timestamp_bits = BITS_PER_LONG - EVICTION_SHIFT;
519 max_order = fls_long(totalram_pages - 1);
520 if (max_order > timestamp_bits)
521 bucket_order = max_order - timestamp_bits;
522 pr_info("workingset: timestamp_bits=%d max_order=%d bucket_order=%u\n",
523 timestamp_bits, max_order, bucket_order);
524
525 ret = prealloc_shrinker(&workingset_shadow_shrinker);
526 if (ret)
527 goto err;
528 ret = __list_lru_init(&shadow_nodes, true, &shadow_nodes_key,
529 &workingset_shadow_shrinker);
530 if (ret)
531 goto err_list_lru;
532 register_shrinker_prepared(&workingset_shadow_shrinker);
533 return 0;
534 err_list_lru:
535 free_prealloced_shrinker(&workingset_shadow_shrinker);
536 err:
537 return ret;
538 }
539 module_init(workingset_init);
540
