workingset.c

  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/mm_inline.h>
 10#include <linux/writeback.h>
 11#include <linux/shmem_fs.h>
 12#include <linux/pagemap.h>
 13#include <linux/atomic.h>
 14#include <linux/module.h>
 15#include <linux/swap.h>
 16#include <linux/dax.h>
 17#include <linux/fs.h>
 18#include <linux/mm.h>
 19#include "internal.h"
 20
 21/*
 22 *		Double CLOCK lists
 23 *
 24 * Per node, two clock lists are maintained for file pages: the
 25 * inactive and the active list.  Freshly faulted pages start out at
 26 * the head of the inactive list and page reclaim scans pages from the
 27 * tail.  Pages that are accessed multiple times on the inactive list
 28 * are promoted to the active list, to protect them from reclaim,
 29 * whereas active pages are demoted to the inactive list when the
 30 * active list grows too big.
 31 *
 32 *   fault ------------------------+
 33 *                                 |
 34 *              +--------------+   |            +-------------+
 35 *   reclaim <- |   inactive   | <-+-- demotion |    active   | <--+
 36 *              +--------------+                +-------------+    |
 37 *                     |                                           |
 38 *                     +-------------- promotion ------------------+
 39 *
 40 *
 41 *		Access frequency and refault distance
 42 *
 43 * A workload is thrashing when its pages are frequently used but they
 44 * are evicted from the inactive list every time before another access
 45 * would have promoted them to the active list.
 46 *
 47 * In cases where the average access distance between thrashing pages
 48 * is bigger than the size of memory there is nothing that can be
 49 * done - the thrashing set could never fit into memory under any
 50 * circumstance.
 51 *
 52 * However, the average access distance could be bigger than the
 53 * inactive list, yet smaller than the size of memory.  In this case,
 54 * the set could fit into memory if it weren't for the currently
 55 * active pages - which may be used more, hopefully less frequently:
 56 *
 57 *      +-memory available to cache-+
 58 *      |                           |
 59 *      +-inactive------+-active----+
 60 *  a b | c d e f g h i | J K L M N |
 61 *      +---------------+-----------+
 62 *
 63 * It is prohibitively expensive to accurately track access frequency
 64 * of pages.  But a reasonable approximation can be made to measure
 65 * thrashing on the inactive list, after which refaulting pages can be
 66 * activated optimistically to compete with the existing active pages.
 67 *
 68 * Approximating inactive page access frequency - Observations:
 69 *
 70 * 1. When a page is accessed for the first time, it is added to the
 71 *    head of the inactive list, slides every existing inactive page
 72 *    towards the tail by one slot, and pushes the current tail page
 73 *    out of memory.
 74 *
 75 * 2. When a page is accessed for the second time, it is promoted to
 76 *    the active list, shrinking the inactive list by one slot.  This
 77 *    also slides all inactive pages that were faulted into the cache
 78 *    more recently than the activated page towards the tail of the
 79 *    inactive list.
 80 *
 81 * Thus:
 82 *
 83 * 1. The sum of evictions and activations between any two points in
 84 *    time indicate the minimum number of inactive pages accessed in
 85 *    between.
 86 *
 87 * 2. Moving one inactive page N page slots towards the tail of the
 88 *    list requires at least N inactive page accesses.
 89 *
 90 * Combining these:
 91 *
 92 * 1. When a page is finally evicted from memory, the number of
 93 *    inactive pages accessed while the page was in cache is at least
 94 *    the number of page slots on the inactive list.
 95 *
 96 * 2. In addition, measuring the sum of evictions and activations (E)
 97 *    at the time of a page's eviction, and comparing it to another
 98 *    reading (R) at the time the page faults back into memory tells
 99 *    the minimum number of accesses while the page was not cached.
100 *    This is called the refault distance.
101 *
102 * Because the first access of the page was the fault and the second
103 * access the refault, we combine the in-cache distance with the
104 * out-of-cache distance to get the complete minimum access distance
105 * of this page:
106 *
107 *      NR_inactive + (R - E)
108 *
109 * And knowing the minimum access distance of a page, we can easily
110 * tell if the page would be able to stay in cache assuming all page
111 * slots in the cache were available:
112 *
113 *   NR_inactive + (R - E) <= NR_inactive + NR_active
114 *
115 * If we have swap we should consider about NR_inactive_anon and
116 * NR_active_anon, so for page cache and anonymous respectively:
117 *
118 *   NR_inactive_file + (R - E) <= NR_inactive_file + NR_active_file
119 *   + NR_inactive_anon + NR_active_anon
120 *
121 *   NR_inactive_anon + (R - E) <= NR_inactive_anon + NR_active_anon
122 *   + NR_inactive_file + NR_active_file
123 *
124 * Which can be further simplified to:
125 *
126 *   (R - E) <= NR_active_file + NR_inactive_anon + NR_active_anon
127 *
128 *   (R - E) <= NR_active_anon + NR_inactive_file + NR_active_file
129 *
130 * Put into words, the refault distance (out-of-cache) can be seen as
131 * a deficit in inactive list space (in-cache).  If the inactive list
132 * had (R - E) more page slots, the page would not have been evicted
133 * in between accesses, but activated instead.  And on a full system,
134 * the only thing eating into inactive list space is active pages.
135 *
136 *
137 *		Refaulting inactive pages
138 *
139 * All that is known about the active list is that the pages have been
140 * accessed more than once in the past.  This means that at any given
141 * time there is actually a good chance that pages on the active list
142 * are no longer in active use.
143 *
144 * So when a refault distance of (R - E) is observed and there are at
145 * least (R - E) pages in the userspace workingset, the refaulting page
146 * is activated optimistically in the hope that (R - E) pages are actually
147 * used less frequently than the refaulting page - or even not used at
148 * all anymore.
149 *
150 * That means if inactive cache is refaulting with a suitable refault
151 * distance, we assume the cache workingset is transitioning and put
152 * pressure on the current workingset.
153 *
154 * If this is wrong and demotion kicks in, the pages which are truly
155 * used more frequently will be reactivated while the less frequently
156 * used once will be evicted from memory.
157 *
158 * But if this is right, the stale pages will be pushed out of memory
159 * and the used pages get to stay in cache.
160 *
161 *		Refaulting active pages
162 *
163 * If on the other hand the refaulting pages have recently been
164 * deactivated, it means that the active list is no longer protecting
165 * actively used cache from reclaim. The cache is NOT transitioning to
166 * a different workingset; the existing workingset is thrashing in the
167 * space allocated to the page cache.
168 *
169 *
170 *		Implementation
171 *
172 * For each node's LRU lists, a counter for inactive evictions and
173 * activations is maintained (node->nonresident_age).
174 *
175 * On eviction, a snapshot of this counter (along with some bits to
176 * identify the node) is stored in the now empty page cache
177 * slot of the evicted page.  This is called a shadow entry.
178 *
179 * On cache misses for which there are shadow entries, an eligible
180 * refault distance will immediately activate the refaulting page.
181 */
182
183#define WORKINGSET_SHIFT 1
184#define EVICTION_SHIFT	((BITS_PER_LONG - BITS_PER_XA_VALUE) +	\
185			 WORKINGSET_SHIFT + NODES_SHIFT + \
186			 MEM_CGROUP_ID_SHIFT)
187#define EVICTION_MASK	(~0UL >> EVICTION_SHIFT)
188
189/*
190 * Eviction timestamps need to be able to cover the full range of
191 * actionable refaults. However, bits are tight in the xarray
192 * entry, and after storing the identifier for the lruvec there might
193 * not be enough left to represent every single actionable refault. In
194 * that case, we have to sacrifice granularity for distance, and group
195 * evictions into coarser buckets by shaving off lower timestamp bits.
196 */
197static unsigned int bucket_order __read_mostly;
198
199static void *pack_shadow(int memcgid, pg_data_t *pgdat, unsigned long eviction,
200			 bool workingset)
201{
202	eviction &= EVICTION_MASK;
203	eviction = (eviction << MEM_CGROUP_ID_SHIFT) | memcgid;
204	eviction = (eviction << NODES_SHIFT) | pgdat->node_id;
205	eviction = (eviction << WORKINGSET_SHIFT) | workingset;
206
207	return xa_mk_value(eviction);
208}
209
210static void unpack_shadow(void *shadow, int *memcgidp, pg_data_t **pgdat,
211			  unsigned long *evictionp, bool *workingsetp)
212{
213	unsigned long entry = xa_to_value(shadow);
214	int memcgid, nid;
215	bool workingset;
216
217	workingset = entry & ((1UL << WORKINGSET_SHIFT) - 1);
218	entry >>= WORKINGSET_SHIFT;
219	nid = entry & ((1UL << NODES_SHIFT) - 1);
220	entry >>= NODES_SHIFT;
221	memcgid = entry & ((1UL << MEM_CGROUP_ID_SHIFT) - 1);
222	entry >>= MEM_CGROUP_ID_SHIFT;
223
224	*memcgidp = memcgid;
225	*pgdat = NODE_DATA(nid);
226	*evictionp = entry;
227	*workingsetp = workingset;
228}
229
230#ifdef CONFIG_LRU_GEN
231
232static void *lru_gen_eviction(struct folio *folio)
233{
234	int hist;
235	unsigned long token;
236	unsigned long min_seq;
237	struct lruvec *lruvec;
238	struct lru_gen_folio *lrugen;
239	int type = folio_is_file_lru(folio);
240	int delta = folio_nr_pages(folio);
241	int refs = folio_lru_refs(folio);
242	bool workingset = folio_test_workingset(folio);
243	int tier = lru_tier_from_refs(refs, workingset);
244	struct mem_cgroup *memcg = folio_memcg(folio);
245	struct pglist_data *pgdat = folio_pgdat(folio);
246
247	BUILD_BUG_ON(LRU_GEN_WIDTH + LRU_REFS_WIDTH > BITS_PER_LONG - EVICTION_SHIFT);
248
249	lruvec = mem_cgroup_lruvec(memcg, pgdat);
250	lrugen = &lruvec->lrugen;
251	min_seq = READ_ONCE(lrugen->min_seq[type]);
252	token = (min_seq << LRU_REFS_WIDTH) | max(refs - 1, 0);
253
254	hist = lru_hist_from_seq(min_seq);
255	atomic_long_add(delta, &lrugen->evicted[hist][type][tier]);
256
257	return pack_shadow(mem_cgroup_id(memcg), pgdat, token, workingset);
258}
259
260/*
261 * Tests if the shadow entry is for a folio that was recently evicted.
262 * Fills in @lruvec, @token, @workingset with the values unpacked from shadow.
263 */
264static bool lru_gen_test_recent(void *shadow, struct lruvec **lruvec,
265				unsigned long *token, bool *workingset)
266{
267	int memcg_id;
268	unsigned long max_seq;
269	struct mem_cgroup *memcg;
270	struct pglist_data *pgdat;
271
272	unpack_shadow(shadow, &memcg_id, &pgdat, token, workingset);
273
274	memcg = mem_cgroup_from_id(memcg_id);
275	*lruvec = mem_cgroup_lruvec(memcg, pgdat);
276
277	max_seq = READ_ONCE((*lruvec)->lrugen.max_seq);
278	max_seq &= EVICTION_MASK >> LRU_REFS_WIDTH;
279
280	return abs_diff(max_seq, *token >> LRU_REFS_WIDTH) < MAX_NR_GENS;
281}
282
283static void lru_gen_refault(struct folio *folio, void *shadow)
284{
285	bool recent;
286	int hist, tier, refs;
287	bool workingset;
288	unsigned long token;
289	struct lruvec *lruvec;
290	struct lru_gen_folio *lrugen;
291	int type = folio_is_file_lru(folio);
292	int delta = folio_nr_pages(folio);
293
294	rcu_read_lock();
295
296	recent = lru_gen_test_recent(shadow, &lruvec, &token, &workingset);
297	if (lruvec != folio_lruvec(folio))
298		goto unlock;
299
300	mod_lruvec_state(lruvec, WORKINGSET_REFAULT_BASE + type, delta);
301
302	if (!recent)
303		goto unlock;
304
305	lrugen = &lruvec->lrugen;
306
307	hist = lru_hist_from_seq(READ_ONCE(lrugen->min_seq[type]));
308	refs = (token & (BIT(LRU_REFS_WIDTH) - 1)) + 1;
309	tier = lru_tier_from_refs(refs, workingset);
310
311	atomic_long_add(delta, &lrugen->refaulted[hist][type][tier]);
312
313	/* see folio_add_lru() where folio_set_active() will be called */
314	if (lru_gen_in_fault())
315		mod_lruvec_state(lruvec, WORKINGSET_ACTIVATE_BASE + type, delta);
316
317	if (workingset) {
318		folio_set_workingset(folio);
319		mod_lruvec_state(lruvec, WORKINGSET_RESTORE_BASE + type, delta);
320	} else
321		set_mask_bits(&folio->flags.f, LRU_REFS_MASK, (refs - 1UL) << LRU_REFS_PGOFF);
322unlock:
323	rcu_read_unlock();
324}
325
326#else /* !CONFIG_LRU_GEN */
327
328static void *lru_gen_eviction(struct folio *folio)
329{
330	return NULL;
331}
332
333static bool lru_gen_test_recent(void *shadow, struct lruvec **lruvec,
334				unsigned long *token, bool *workingset)
335{
336	return false;
337}
338
339static void lru_gen_refault(struct folio *folio, void *shadow)
340{
341}
342
343#endif /* CONFIG_LRU_GEN */
344
345/**
346 * workingset_age_nonresident - age non-resident entries as LRU ages
347 * @lruvec: the lruvec that was aged
348 * @nr_pages: the number of pages to count
349 *
350 * As in-memory pages are aged, non-resident pages need to be aged as
351 * well, in order for the refault distances later on to be comparable
352 * to the in-memory dimensions. This function allows reclaim and LRU
353 * operations to drive the non-resident aging along in parallel.
354 */
355void workingset_age_nonresident(struct lruvec *lruvec, unsigned long nr_pages)
356{
357	/*
358	 * Reclaiming a cgroup means reclaiming all its children in a
359	 * round-robin fashion. That means that each cgroup has an LRU
360	 * order that is composed of the LRU orders of its child
361	 * cgroups; and every page has an LRU position not just in the
362	 * cgroup that owns it, but in all of that group's ancestors.
363	 *
364	 * So when the physical inactive list of a leaf cgroup ages,
365	 * the virtual inactive lists of all its parents, including
366	 * the root cgroup's, age as well.
367	 */
368	do {
369		atomic_long_add(nr_pages, &lruvec->nonresident_age);
370	} while ((lruvec = parent_lruvec(lruvec)));
371}
372
373/**
374 * workingset_eviction - note the eviction of a folio from memory
375 * @target_memcg: the cgroup that is causing the reclaim
376 * @folio: the folio being evicted
377 *
378 * Return: a shadow entry to be stored in @folio->mapping->i_pages in place
379 * of the evicted @folio so that a later refault can be detected.
380 */
381void *workingset_eviction(struct folio *folio, struct mem_cgroup *target_memcg)
382{
383	struct pglist_data *pgdat = folio_pgdat(folio);
384	unsigned long eviction;
385	struct lruvec *lruvec;
386	int memcgid;
387
388	/* Folio is fully exclusive and pins folio's memory cgroup pointer */
389	VM_BUG_ON_FOLIO(folio_test_lru(folio), folio);
390	VM_BUG_ON_FOLIO(folio_ref_count(folio), folio);
391	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
392
393	if (lru_gen_enabled())
394		return lru_gen_eviction(folio);
395
396	lruvec = mem_cgroup_lruvec(target_memcg, pgdat);
397	/* XXX: target_memcg can be NULL, go through lruvec */
398	memcgid = mem_cgroup_id(lruvec_memcg(lruvec));
399	eviction = atomic_long_read(&lruvec->nonresident_age);
400	eviction >>= bucket_order;
401	workingset_age_nonresident(lruvec, folio_nr_pages(folio));
402	return pack_shadow(memcgid, pgdat, eviction,
403				folio_test_workingset(folio));
404}
405
406/**
407 * workingset_test_recent - tests if the shadow entry is for a folio that was
408 * recently evicted. Also fills in @workingset with the value unpacked from
409 * shadow.
410 * @shadow: the shadow entry to be tested.
411 * @file: whether the corresponding folio is from the file lru.
412 * @workingset: where the workingset value unpacked from shadow should
413 * be stored.
414 * @flush: whether to flush cgroup rstat.
415 *
416 * Return: true if the shadow is for a recently evicted folio; false otherwise.
417 */
418bool workingset_test_recent(void *shadow, bool file, bool *workingset,
419				bool flush)
420{
421	struct mem_cgroup *eviction_memcg;
422	struct lruvec *eviction_lruvec;
423	unsigned long refault_distance;
424	unsigned long workingset_size;
425	unsigned long refault;
426	int memcgid;
427	struct pglist_data *pgdat;
428	unsigned long eviction;
429
430	if (lru_gen_enabled()) {
431		bool recent;
432
433		rcu_read_lock();
434		recent = lru_gen_test_recent(shadow, &eviction_lruvec, &eviction, workingset);
435		rcu_read_unlock();
436		return recent;
437	}
438
439	rcu_read_lock();
440	unpack_shadow(shadow, &memcgid, &pgdat, &eviction, workingset);
441	eviction <<= bucket_order;
442
443	/*
444	 * Look up the memcg associated with the stored ID. It might
445	 * have been deleted since the folio's eviction.
446	 *
447	 * Note that in rare events the ID could have been recycled
448	 * for a new cgroup that refaults a shared folio. This is
449	 * impossible to tell from the available data. However, this
450	 * should be a rare and limited disturbance, and activations
451	 * are always speculative anyway. Ultimately, it's the aging
452	 * algorithm's job to shake out the minimum access frequency
453	 * for the active cache.
454	 *
455	 * XXX: On !CONFIG_MEMCG, this will always return NULL; it
456	 * would be better if the root_mem_cgroup existed in all
457	 * configurations instead.
458	 */
459	eviction_memcg = mem_cgroup_from_id(memcgid);
460	if (!mem_cgroup_tryget(eviction_memcg))
461		eviction_memcg = NULL;
462	rcu_read_unlock();
463
464	if (!mem_cgroup_disabled() && !eviction_memcg)
465		return false;
466	/*
467	 * Flush stats (and potentially sleep) outside the RCU read section.
468	 *
469	 * Note that workingset_test_recent() itself might be called in RCU read
470	 * section (for e.g, in cachestat) - these callers need to skip flushing
471	 * stats (via the flush argument).
472	 *
473	 * XXX: With per-memcg flushing and thresholding, is ratelimiting
474	 * still needed here?
475	 */
476	if (flush)
477		mem_cgroup_flush_stats_ratelimited(eviction_memcg);
478
479	eviction_lruvec = mem_cgroup_lruvec(eviction_memcg, pgdat);
480	refault = atomic_long_read(&eviction_lruvec->nonresident_age);
481
482	/*
483	 * Calculate the refault distance
484	 *
485	 * The unsigned subtraction here gives an accurate distance
486	 * across nonresident_age overflows in most cases. There is a
487	 * special case: usually, shadow entries have a short lifetime
488	 * and are either refaulted or reclaimed along with the inode
489	 * before they get too old.  But it is not impossible for the
490	 * nonresident_age to lap a shadow entry in the field, which
491	 * can then result in a false small refault distance, leading
492	 * to a false activation should this old entry actually
493	 * refault again.  However, earlier kernels used to deactivate
494	 * unconditionally with *every* reclaim invocation for the
495	 * longest time, so the occasional inappropriate activation
496	 * leading to pressure on the active list is not a problem.
497	 */
498	refault_distance = (refault - eviction) & EVICTION_MASK;
499
500	/*
501	 * Compare the distance to the existing workingset size. We
502	 * don't activate pages that couldn't stay resident even if
503	 * all the memory was available to the workingset. Whether
504	 * workingset competition needs to consider anon or not depends
505	 * on having free swap space.
506	 */
507	workingset_size = lruvec_page_state(eviction_lruvec, NR_ACTIVE_FILE);
508	if (!file) {
509		workingset_size += lruvec_page_state(eviction_lruvec,
510						     NR_INACTIVE_FILE);
511	}
512	if (mem_cgroup_get_nr_swap_pages(eviction_memcg) > 0) {
513		workingset_size += lruvec_page_state(eviction_lruvec,
514						     NR_ACTIVE_ANON);
515		if (file) {
516			workingset_size += lruvec_page_state(eviction_lruvec,
517						     NR_INACTIVE_ANON);
518		}
519	}
520
521	mem_cgroup_put(eviction_memcg);
522	return refault_distance <= workingset_size;
523}
524
525/**
526 * workingset_refault - Evaluate the refault of a previously evicted folio.
527 * @folio: The freshly allocated replacement folio.
528 * @shadow: Shadow entry of the evicted folio.
529 *
530 * Calculates and evaluates the refault distance of the previously
531 * evicted folio in the context of the node and the memcg whose memory
532 * pressure caused the eviction.
533 */
534void workingset_refault(struct folio *folio, void *shadow)
535{
536	bool file = folio_is_file_lru(folio);
537	struct pglist_data *pgdat;
538	struct mem_cgroup *memcg;
539	struct lruvec *lruvec;
540	bool workingset;
541	long nr;
542
543	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
544
545	if (lru_gen_enabled()) {
546		lru_gen_refault(folio, shadow);
547		return;
548	}
549
550	/*
551	 * The activation decision for this folio is made at the level
552	 * where the eviction occurred, as that is where the LRU order
553	 * during folio reclaim is being determined.
554	 *
555	 * However, the cgroup that will own the folio is the one that
556	 * is actually experiencing the refault event. Make sure the folio is
557	 * locked to guarantee folio_memcg() stability throughout.
558	 */
559	nr = folio_nr_pages(folio);
560	memcg = folio_memcg(folio);
561	pgdat = folio_pgdat(folio);
562	lruvec = mem_cgroup_lruvec(memcg, pgdat);
563
564	mod_lruvec_state(lruvec, WORKINGSET_REFAULT_BASE + file, nr);
565
566	if (!workingset_test_recent(shadow, file, &workingset, true))
567		return;
568
569	folio_set_active(folio);
570	workingset_age_nonresident(lruvec, nr);
571	mod_lruvec_state(lruvec, WORKINGSET_ACTIVATE_BASE + file, nr);
572
573	/* Folio was active prior to eviction */
574	if (workingset) {
575		folio_set_workingset(folio);
576		/*
577		 * XXX: Move to folio_add_lru() when it supports new vs
578		 * putback
579		 */
580		lru_note_cost_refault(folio);
581		mod_lruvec_state(lruvec, WORKINGSET_RESTORE_BASE + file, nr);
582	}
583}
584
585/**
586 * workingset_activation - note a page activation
587 * @folio: Folio that is being activated.
588 */
589void workingset_activation(struct folio *folio)
590{
591	/*
592	 * Filter non-memcg pages here, e.g. unmap can call
593	 * mark_page_accessed() on VDSO pages.
594	 */
595	if (mem_cgroup_disabled() || folio_memcg_charged(folio))
596		workingset_age_nonresident(folio_lruvec(folio), folio_nr_pages(folio));
597}
598
599/*
600 * Shadow entries reflect the share of the working set that does not
601 * fit into memory, so their number depends on the access pattern of
602 * the workload.  In most cases, they will refault or get reclaimed
603 * along with the inode, but a (malicious) workload that streams
604 * through files with a total size several times that of available
605 * memory, while preventing the inodes from being reclaimed, can
606 * create excessive amounts of shadow nodes.  To keep a lid on this,
607 * track shadow nodes and reclaim them when they grow way past the
608 * point where they would still be useful.
609 */
610
611struct list_lru shadow_nodes;
612
613void workingset_update_node(struct xa_node *node)
614{
615	struct page *page = virt_to_page(node);
616
617	/*
618	 * Track non-empty nodes that contain only shadow entries;
619	 * unlink those that contain pages or are being freed.
620	 *
621	 * Avoid acquiring the list_lru lock when the nodes are
622	 * already where they should be. The list_empty() test is safe
623	 * as node->private_list is protected by the i_pages lock.
624	 */
625	lockdep_assert_held(&node->array->xa_lock);
626
627	if (node->count && node->count == node->nr_values) {
628		if (list_empty(&node->private_list)) {
629			list_lru_add_obj(&shadow_nodes, &node->private_list);
630			__inc_node_page_state(page, WORKINGSET_NODES);
631		}
632	} else {
633		if (!list_empty(&node->private_list)) {
634			list_lru_del_obj(&shadow_nodes, &node->private_list);
635			__dec_node_page_state(page, WORKINGSET_NODES);
636		}
637	}
638}
639
640static unsigned long count_shadow_nodes(struct shrinker *shrinker,
641					struct shrink_control *sc)
642{
643	unsigned long max_nodes;
644	unsigned long nodes;
645	unsigned long pages;
646
647	nodes = list_lru_shrink_count(&shadow_nodes, sc);
648	if (!nodes)
649		return SHRINK_EMPTY;
650
651	/*
652	 * Approximate a reasonable limit for the nodes
653	 * containing shadow entries. We don't need to keep more
654	 * shadow entries than possible pages on the active list,
655	 * since refault distances bigger than that are dismissed.
656	 *
657	 * The size of the active list converges toward 100% of
658	 * overall page cache as memory grows, with only a tiny
659	 * inactive list. Assume the total cache size for that.
660	 *
661	 * Nodes might be sparsely populated, with only one shadow
662	 * entry in the extreme case. Obviously, we cannot keep one
663	 * node for every eligible shadow entry, so compromise on a
664	 * worst-case density of 1/8th. Below that, not all eligible
665	 * refaults can be detected anymore.
666	 *
667	 * On 64-bit with 7 xa_nodes per page and 64 slots
668	 * each, this will reclaim shadow entries when they consume
669	 * ~1.8% of available memory:
670	 *
671	 * PAGE_SIZE / xa_nodes / node_entries * 8 / PAGE_SIZE
672	 */
673#ifdef CONFIG_MEMCG
674	if (sc->memcg) {
675		struct lruvec *lruvec;
676		int i;
677
678		mem_cgroup_flush_stats_ratelimited(sc->memcg);
679		lruvec = mem_cgroup_lruvec(sc->memcg, NODE_DATA(sc->nid));
680		for (pages = 0, i = 0; i < NR_LRU_LISTS; i++)
681			pages += lruvec_page_state_local(lruvec,
682							 NR_LRU_BASE + i);
683		pages += lruvec_page_state_local(
684			lruvec, NR_SLAB_RECLAIMABLE_B) >> PAGE_SHIFT;
685		pages += lruvec_page_state_local(
686			lruvec, NR_SLAB_UNRECLAIMABLE_B) >> PAGE_SHIFT;
687	} else
688#endif
689		pages = node_present_pages(sc->nid);
690
691	max_nodes = pages >> (XA_CHUNK_SHIFT - 3);
692
693	if (nodes <= max_nodes)
694		return 0;
695	return nodes - max_nodes;
696}
697
698static enum lru_status shadow_lru_isolate(struct list_head *item,
699					  struct list_lru_one *lru,
700					  void *arg) __must_hold(lru->lock)
701{
702	struct xa_node *node = container_of(item, struct xa_node, private_list);
703	struct address_space *mapping;
704	int ret;
705
706	/*
707	 * Page cache insertions and deletions synchronously maintain
708	 * the shadow node LRU under the i_pages lock and the
709	 * &lru->lock. Because the page cache tree is emptied before
710	 * the inode can be destroyed, holding the &lru->lock pins any
711	 * address_space that has nodes on the LRU.
712	 *
713	 * We can then safely transition to the i_pages lock to
714	 * pin only the address_space of the particular node we want
715	 * to reclaim, take the node off-LRU, and drop the &lru->lock.
716	 */
717
718	mapping = container_of(node->array, struct address_space, i_pages);
719
720	/* Coming from the list, invert the lock order */
721	if (!xa_trylock(&mapping->i_pages)) {
722		spin_unlock_irq(&lru->lock);
723		ret = LRU_RETRY;
724		goto out;
725	}
726
727	/* For page cache we need to hold i_lock */
728	if (mapping->host != NULL) {
729		if (!spin_trylock(&mapping->host->i_lock)) {
730			xa_unlock(&mapping->i_pages);
731			spin_unlock_irq(&lru->lock);
732			ret = LRU_RETRY;
733			goto out;
734		}
735	}
736
737	list_lru_isolate(lru, item);
738	__dec_node_page_state(virt_to_page(node), WORKINGSET_NODES);
739
740	spin_unlock(&lru->lock);
741
742	/*
743	 * The nodes should only contain one or more shadow entries,
744	 * no pages, so we expect to be able to remove them all and
745	 * delete and free the empty node afterwards.
746	 */
747	if (WARN_ON_ONCE(!node->nr_values))
748		goto out_invalid;
749	if (WARN_ON_ONCE(node->count != node->nr_values))
750		goto out_invalid;
751	xa_delete_node(node, workingset_update_node);
752	mod_lruvec_kmem_state(node, WORKINGSET_NODERECLAIM, 1);
753
754out_invalid:
755	xa_unlock_irq(&mapping->i_pages);
756	if (mapping->host != NULL) {
757		if (mapping_shrinkable(mapping))
758			inode_lru_list_add(mapping->host);
759		spin_unlock(&mapping->host->i_lock);
760	}
761	ret = LRU_REMOVED_RETRY;
762out:
763	cond_resched();
764	return ret;
765}
766
767static unsigned long scan_shadow_nodes(struct shrinker *shrinker,
768				       struct shrink_control *sc)
769{
770	/* list_lru lock nests inside the IRQ-safe i_pages lock */
771	return list_lru_shrink_walk_irq(&shadow_nodes, sc, shadow_lru_isolate,
772					NULL);
773}
774
775/*
776 * Our list_lru->lock is IRQ-safe as it nests inside the IRQ-safe
777 * i_pages lock.
778 */
779static struct lock_class_key shadow_nodes_key;
780
781static int __init workingset_init(void)
782{
783	struct shrinker *workingset_shadow_shrinker;
784	unsigned int timestamp_bits;
785	unsigned int max_order;
786	int ret = -ENOMEM;
787
788	BUILD_BUG_ON(BITS_PER_LONG < EVICTION_SHIFT);
789	/*
790	 * Calculate the eviction bucket size to cover the longest
791	 * actionable refault distance, which is currently half of
792	 * memory (totalram_pages/2). However, memory hotplug may add
793	 * some more pages at runtime, so keep working with up to
794	 * double the initial memory by using totalram_pages as-is.
795	 */
796	timestamp_bits = BITS_PER_LONG - EVICTION_SHIFT;
797	max_order = fls_long(totalram_pages() - 1);
798	if (max_order > timestamp_bits)
799		bucket_order = max_order - timestamp_bits;
800	pr_info("workingset: timestamp_bits=%d max_order=%d bucket_order=%u\n",
801	       timestamp_bits, max_order, bucket_order);
802
803	workingset_shadow_shrinker = shrinker_alloc(SHRINKER_NUMA_AWARE |
804						    SHRINKER_MEMCG_AWARE,
805						    "mm-shadow");
806	if (!workingset_shadow_shrinker)
807		goto err;
808
809	ret = list_lru_init_memcg_key(&shadow_nodes, workingset_shadow_shrinker,
810				      &shadow_nodes_key);
811	if (ret)
812		goto err_list_lru;
813
814	workingset_shadow_shrinker->count_objects = count_shadow_nodes;
815	workingset_shadow_shrinker->scan_objects = scan_shadow_nodes;
816	/* ->count reports only fully expendable nodes */
817	workingset_shadow_shrinker->seeks = 0;
818
819	shrinker_register(workingset_shadow_shrinker);
820	return 0;
821err_list_lru:
822	shrinker_free(workingset_shadow_shrinker);
823err:
824	return ret;
825}
826module_init(workingset_init);