mm/mglru: rework workingset protection

With the aging feedback no longer considering the distribution of folios
in each generation, rework workingset protection to better distribute
folios across MAX_NR_GENS.  This is achieved by reusing PG_workingset and
PG_referenced/LRU_REFS_FLAGS in a slightly different way.

For folios accessed multiple times through file descriptors, make
lru_gen_inc_refs() set additional bits of LRU_REFS_WIDTH in folio->flags
after PG_referenced, then PG_workingset after LRU_REFS_WIDTH.  After all
its bits are set, i.e., LRU_REFS_FLAGS|BIT(PG_workingset), a folio is
lazily promoted into the second oldest generation in the eviction path. 
And when folio_inc_gen() does that, it clears LRU_REFS_FLAGS so that
lru_gen_inc_refs() can start over.  For this case, LRU_REFS_MASK is only
valid when PG_referenced is set.

For folios accessed multiple times through page tables, folio_update_gen()
from a page table walk or lru_gen_set_refs() from a rmap walk sets
PG_referenced after the accessed bit is cleared for the first time. 
Thereafter, those two paths set PG_workingset and promote folios to the
youngest generation.  Like folio_inc_gen(), when folio_update_gen() does
that, it also clears PG_referenced.  For this case, LRU_REFS_MASK is not
used.

For both of the cases, after PG_workingset is set on a folio, it remains
until this folio is either reclaimed, or "deactivated" by
lru_gen_clear_refs().  It can be set again if lru_gen_test_recent()
returns true upon a refault.

When adding folios to the LRU lists, lru_gen_folio_seq() distributes
them as follows:
+---------------------------------+---------------------------------+
|    Accessed thru page tables    | Accessed thru file descriptors  |
+---------------------------------+---------------------------------+
| PG_active (set while isolated)  |                                 |
+----------------+----------------+----------------+----------------+
| PG_workingset  | PG_referenced  | PG_workingset  | LRU_REFS_FLAGS |
+---------------------------------+---------------------------------+
|<--------- MIN_NR_GENS --------->|                                 |
|<-------------------------- MAX_NR_GENS -------------------------->|

After this patch, some typical client and server workloads showed
improvements under heavy memory pressure.  For example, Python TPC-C,
which was used to benchmark a different approach [1] to better detect
refault distances, showed a significant decrease in total refaults:

                            Before      After      Change
  Time (seconds)            10801       10801      0%
  Executed (transactions)   41472       43663      +5%
  workingset_nodes          109070      120244     +10%
  workingset_refault_anon   5019627     7281831    +45%
  workingset_refault_file   1294678786  554855564  -57%
  workingset_refault_total  1299698413  562137395  -57%

[1] https://lore.kernel.org/20230920190244.16839-1-ryncsn@gmail.com/

Link: https://lkml.kernel.org/r/20241231043538.4075764-7-yuzhao@google.com
Signed-off-by: Yu Zhao <yuzhao@google.com>
Reported-by: Kairui Song <kasong@tencent.com>
Closes: https://lore.kernel.org/CAOUHufahuWcKf5f1Sg3emnqX+cODuR=2TQo7T4Gr-QYLujn4RA@mail.gmail.com/
Tested-by: Kalesh Singh <kaleshsingh@google.com>
Cc: Barry Song <v-songbaohua@oppo.com>
Cc: Bharata B Rao <bharata@amd.com>
Cc: David Stevens <stevensd@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 85 insertions, 58 deletions

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 761298d9c09a..9b8e0a9fc9d5 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -862,6 +862,31 @@ enum folio_references {
 	FOLIOREF_ACTIVATE,
 };
 
+#ifdef CONFIG_LRU_GEN
+/*
+ * Only used on a mapped folio in the eviction (rmap walk) path, where promotion
+ * needs to be done by taking the folio off the LRU list and then adding it back
+ * with PG_active set. In contrast, the aging (page table walk) path uses
+ * folio_update_gen().
+ */
+static bool lru_gen_set_refs(struct folio *folio)
+{
+	/* see the comment on LRU_REFS_FLAGS */
+	if (!folio_test_referenced(folio) && !folio_test_workingset(folio)) {
+		set_mask_bits(&folio->flags, LRU_REFS_MASK, BIT(PG_referenced));
+		return false;
+	}
+
+	set_mask_bits(&folio->flags, LRU_REFS_FLAGS, BIT(PG_workingset));
+	return true;
+}
+#else
+static bool lru_gen_set_refs(struct folio *folio)
+{
+	return false;
+}
+#endif /* CONFIG_LRU_GEN */
+
 static enum folio_references folio_check_references(struct folio *folio,
 						  struct scan_control *sc)
 {
@@ -870,7 +895,6 @@ static enum folio_references folio_check_references(struct folio *folio,
 
 	referenced_ptes = folio_referenced(folio, 1, sc->target_mem_cgroup,
 					   &vm_flags);
-	referenced_folio = folio_test_clear_referenced(folio);
 
 	/*
 	 * The supposedly reclaimable folio was found to be in a VM_LOCKED vma.
@@ -888,6 +912,15 @@ static enum folio_references folio_check_references(struct folio *folio,
 	if (referenced_ptes == -1)
 		return FOLIOREF_KEEP;
 
+	if (lru_gen_enabled()) {
+		if (!referenced_ptes)
+			return FOLIOREF_RECLAIM;
+
+		return lru_gen_set_refs(folio) ? FOLIOREF_ACTIVATE : FOLIOREF_KEEP;
+	}
+
+	referenced_folio = folio_test_clear_referenced(folio);
+
 	if (referenced_ptes) {
 		/*
 		 * All mapped folios start out with page table
@@ -1092,11 +1125,6 @@ retry:
 		if (!sc->may_unmap && folio_mapped(folio))
 			goto keep_locked;
 
-		/* folio_update_gen() tried to promote this page? */
-		if (lru_gen_enabled() && !ignore_references &&
-		    folio_mapped(folio) && folio_test_referenced(folio))
-			goto keep_locked;
-
 		/*
 		 * The number of dirty pages determines if a node is marked
 		 * reclaim_congested. kswapd will stall and start writing
@@ -3159,16 +3187,19 @@ static int folio_update_gen(struct folio *folio, int gen)
 
 	VM_WARN_ON_ONCE(gen >= MAX_NR_GENS);
 
+	/* see the comment on LRU_REFS_FLAGS */
+	if (!folio_test_referenced(folio) && !folio_test_workingset(folio)) {
+		set_mask_bits(&folio->flags, LRU_REFS_MASK, BIT(PG_referenced));
+		return -1;
+	}
+
 	do {
 		/* lru_gen_del_folio() has isolated this page? */
-		if (!(old_flags & LRU_GEN_MASK)) {
-			/* for shrink_folio_list() */
-			new_flags = old_flags | BIT(PG_referenced);
-			continue;
-		}
+		if (!(old_flags & LRU_GEN_MASK))
+			return -1;
 
-		new_flags = old_flags & ~(LRU_GEN_MASK | LRU_REFS_MASK | LRU_REFS_FLAGS);
-		new_flags |= (gen + 1UL) << LRU_GEN_PGOFF;
+		new_flags = old_flags & ~(LRU_GEN_MASK | LRU_REFS_FLAGS);
+		new_flags |= ((gen + 1UL) << LRU_GEN_PGOFF) | BIT(PG_workingset);
 	} while (!try_cmpxchg(&folio->flags, &old_flags, new_flags));
 
 	return ((old_flags & LRU_GEN_MASK) >> LRU_GEN_PGOFF) - 1;
@@ -3192,7 +3223,7 @@ static int folio_inc_gen(struct lruvec *lruvec, struct folio *folio, bool reclai
 
 		new_gen = (old_gen + 1) % MAX_NR_GENS;
 
-		new_flags = old_flags & ~(LRU_GEN_MASK | LRU_REFS_MASK | LRU_REFS_FLAGS);
+		new_flags = old_flags & ~(LRU_GEN_MASK | LRU_REFS_FLAGS);
 		new_flags |= (new_gen + 1UL) << LRU_GEN_PGOFF;
 		/* for folio_end_writeback() */
 		if (reclaiming)
@@ -3370,9 +3401,11 @@ static unsigned long get_pmd_pfn(pmd_t pmd, struct vm_area_struct *vma, unsigned
 static struct folio *get_pfn_folio(unsigned long pfn, struct mem_cgroup *memcg,
 				   struct pglist_data *pgdat)
 {
-	struct folio *folio;
+	struct folio *folio = pfn_folio(pfn);
+
+	if (folio_lru_gen(folio) < 0)
+		return NULL;
 
-	folio = pfn_folio(pfn);
 	if (folio_nid(folio) != pgdat->node_id)
 		return NULL;
 
@@ -3749,8 +3782,7 @@ static bool inc_min_seq(struct lruvec *lruvec, int type, int swappiness)
 		while (!list_empty(head)) {
 			struct folio *folio = lru_to_folio(head);
 			int refs = folio_lru_refs(folio);
-			int tier = lru_tier_from_refs(refs);
-			int delta = folio_nr_pages(folio);
+			bool workingset = folio_test_workingset(folio);
 
 			VM_WARN_ON_ONCE_FOLIO(folio_test_unevictable(folio), folio);
 			VM_WARN_ON_ONCE_FOLIO(folio_test_active(folio), folio);
@@ -3760,8 +3792,14 @@ static bool inc_min_seq(struct lruvec *lruvec, int type, int swappiness)
 			new_gen = folio_inc_gen(lruvec, folio, false);
 			list_move_tail(&folio->lru, &lrugen->folios[new_gen][type][zone]);
 
-			WRITE_ONCE(lrugen->protected[hist][type][tier],
-				   lrugen->protected[hist][type][tier] + delta);
+			/* don't count the workingset being lazily promoted */
+			if (refs + workingset != BIT(LRU_REFS_WIDTH) + 1) {
+				int tier = lru_tier_from_refs(refs, workingset);
+				int delta = folio_nr_pages(folio);
+
+				WRITE_ONCE(lrugen->protected[hist][type][tier],
+					   lrugen->protected[hist][type][tier] + delta);
+			}
 
 			if (!--remaining)
 				return false;
@@ -4147,16 +4185,10 @@ bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
 			old_gen = folio_update_gen(folio, new_gen);
 			if (old_gen >= 0 && old_gen != new_gen)
 				update_batch_size(walk, folio, old_gen, new_gen);
-
-			continue;
-		}
-
-		old_gen = folio_lru_gen(folio);
-		if (old_gen < 0)
-			folio_set_referenced(folio);
-		else if (old_gen != new_gen) {
-			folio_clear_lru_refs(folio);
-			folio_activate(folio);
+		} else if (lru_gen_set_refs(folio)) {
+			old_gen = folio_lru_gen(folio);
+			if (old_gen >= 0 && old_gen != new_gen)
+				folio_activate(folio);
 		}
 	}
 
@@ -4317,7 +4349,8 @@ static bool sort_folio(struct lruvec *lruvec, struct folio *folio, struct scan_c
 	int zone = folio_zonenum(folio);
 	int delta = folio_nr_pages(folio);
 	int refs = folio_lru_refs(folio);
-	int tier = lru_tier_from_refs(refs);
+	bool workingset = folio_test_workingset(folio);
+	int tier = lru_tier_from_refs(refs, workingset);
 	struct lru_gen_folio *lrugen = &lruvec->lrugen;
 
 	VM_WARN_ON_ONCE_FOLIO(gen >= MAX_NR_GENS, folio);
@@ -4339,14 +4372,17 @@ static bool sort_folio(struct lruvec *lruvec, struct folio *folio, struct scan_c
 	}
 
 	/* protected */
-	if (tier > tier_idx || refs == BIT(LRU_REFS_WIDTH)) {
-		int hist = lru_hist_from_seq(lrugen->min_seq[type]);
-
+	if (tier > tier_idx || refs + workingset == BIT(LRU_REFS_WIDTH) + 1) {
 		gen = folio_inc_gen(lruvec, folio, false);
-		list_move_tail(&folio->lru, &lrugen->folios[gen][type][zone]);
+		list_move(&folio->lru, &lrugen->folios[gen][type][zone]);
+
+		/* don't count the workingset being lazily promoted */
+		if (refs + workingset != BIT(LRU_REFS_WIDTH) + 1) {
+			int hist = lru_hist_from_seq(lrugen->min_seq[type]);
 
-		WRITE_ONCE(lrugen->protected[hist][type][tier],
-			   lrugen->protected[hist][type][tier] + delta);
+			WRITE_ONCE(lrugen->protected[hist][type][tier],
+				   lrugen->protected[hist][type][tier] + delta);
+		}
 		return true;
 	}
 
@@ -4366,8 +4402,7 @@ static bool sort_folio(struct lruvec *lruvec, struct folio *folio, struct scan_c
 	}
 
 	/* waiting for writeback */
-	if (folio_test_locked(folio) || writeback ||
-	    (type == LRU_GEN_FILE && dirty)) {
+	if (writeback || (type == LRU_GEN_FILE && dirty)) {
 		gen = folio_inc_gen(lruvec, folio, true);
 		list_move(&folio->lru, &lrugen->folios[gen][type][zone]);
 		return true;
@@ -4396,13 +4431,12 @@ static bool isolate_folio(struct lruvec *lruvec, struct folio *folio, struct sca
 		return false;
 	}
 
-	/* see the comment on MAX_NR_TIERS */
+	/* see the comment on LRU_REFS_FLAGS */
 	if (!folio_test_referenced(folio))
-		folio_clear_lru_refs(folio);
+		set_mask_bits(&folio->flags, LRU_REFS_MASK, 0);
 
 	/* for shrink_folio_list() */
 	folio_clear_reclaim(folio);
-	folio_clear_referenced(folio);
 
 	success = lru_gen_del_folio(lruvec, folio, true);
 	VM_WARN_ON_ONCE_FOLIO(!success, folio);
@@ -4592,31 +4626,24 @@ retry:
 			type ? LRU_INACTIVE_FILE : LRU_INACTIVE_ANON);
 
 	list_for_each_entry_safe_reverse(folio, next, &list, lru) {
+		DEFINE_MIN_SEQ(lruvec);
+
 		if (!folio_evictable(folio)) {
 			list_del(&folio->lru);
 			folio_putback_lru(folio);
 			continue;
 		}
 
-		if (folio_test_reclaim(folio) &&
-		    (folio_test_dirty(folio) || folio_test_writeback(folio))) {
-			/* restore LRU_REFS_FLAGS cleared by isolate_folio() */
-			if (folio_test_workingset(folio))
-				folio_set_referenced(folio);
-			continue;
-		}
-
-		if (skip_retry || folio_test_active(folio) || folio_test_referenced(folio) ||
-		    folio_mapped(folio) || folio_test_locked(folio) ||
-		    folio_test_dirty(folio) || folio_test_writeback(folio)) {
-			/* don't add rejected folios to the oldest generation */
-			set_mask_bits(&folio->flags, LRU_REFS_MASK | LRU_REFS_FLAGS,
-				      BIT(PG_active));
+		/* retry folios that may have missed folio_rotate_reclaimable() */
+		if (!skip_retry && !folio_test_active(folio) && !folio_mapped(folio) &&
+		    !folio_test_dirty(folio) && !folio_test_writeback(folio)) {
+			list_move(&folio->lru, &clean);
 			continue;
 		}
 
-		/* retry folios that may have missed folio_rotate_reclaimable() */
-		list_move(&folio->lru, &clean);
+		/* don't add rejected folios to the oldest generation */
+		if (lru_gen_folio_seq(lruvec, folio, false) == min_seq[type])
+			set_mask_bits(&folio->flags, LRU_REFS_FLAGS, BIT(PG_active));
 	}
 
 	spin_lock_irq(&lruvec->lru_lock);