[PATCH mm-unstable v3 5/6] mm/mglru: rework refault detection

[Yu Zhao <yuzhao@google.com>]

With anon and file min_seq being able to move independently, rework
workingset protection as well so that the comparison of refaults
between anon and file is always on an equal footing.

Specifically, make lru_gen_test_recent() return true for refaults
happening within the distance of MAX_NR_GENS. For example, if min_seq
of a type is max_seq-MIN_NR_GENS, refaults from min_seq-1, i.e.,
max_seq-MIN_NR_GENS-1, are also considered recent, since the distance
max_seq-(max_seq-MIN_NR_GENS-1), i.e., MIN_NR_GENS+1 is less than
MAX_NR_GENS.

As an intermediate step to the final optimization, this change by
itself should not have userspace-visiable effects beyond performance.

Reported-by: Kairui Song <kasong@tencent.com>
Closes: https://lore.kernel.org/CAOUHufahuWcKf5f1Sg3emnqX+cODuR=2TQo7T4Gr-QYLujn4RA@mail.gmail.com/
Signed-off-by: Yu Zhao <yuzhao@google.com>
Tested-by: Kalesh Singh <kaleshsingh@google.com>
---
 mm/workingset.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/mm/workingset.c b/mm/workingset.c
index ad181d1b8cf1..2c310c29f51e 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -260,11 +260,11 @@ static void *lru_gen_eviction(struct folio *folio)
  * Tests if the shadow entry is for a folio that was recently evicted.
  * Fills in @lruvec, @token, @workingset with the values unpacked from shadow.
  */
-static bool lru_gen_test_recent(void *shadow, bool file, struct lruvec **lruvec,
+static bool lru_gen_test_recent(void *shadow, struct lruvec **lruvec,
 				unsigned long *token, bool *workingset)
 {
 	int memcg_id;
-	unsigned long min_seq;
+	unsigned long max_seq;
 	struct mem_cgroup *memcg;
 	struct pglist_data *pgdat;
 
@@ -273,8 +273,10 @@ static bool lru_gen_test_recent(void *shadow, bool file, struct lruvec **lruvec,
 	memcg = mem_cgroup_from_id(memcg_id);
 	*lruvec = mem_cgroup_lruvec(memcg, pgdat);
 
-	min_seq = READ_ONCE((*lruvec)->lrugen.min_seq[file]);
-	return (*token >> LRU_REFS_WIDTH) == (min_seq & (EVICTION_MASK >> LRU_REFS_WIDTH));
+	max_seq = READ_ONCE((*lruvec)->lrugen.max_seq);
+	max_seq &= EVICTION_MASK >> LRU_REFS_WIDTH;
+
+	return abs_diff(max_seq, *token >> LRU_REFS_WIDTH) < MAX_NR_GENS;
 }
 
 static void lru_gen_refault(struct folio *folio, void *shadow)
@@ -290,7 +292,7 @@ static void lru_gen_refault(struct folio *folio, void *shadow)
 
 	rcu_read_lock();
 
-	recent = lru_gen_test_recent(shadow, type, &lruvec, &token, &workingset);
+	recent = lru_gen_test_recent(shadow, &lruvec, &token, &workingset);
 	if (lruvec != folio_lruvec(folio))
 		goto unlock;
 
@@ -331,7 +333,7 @@ static void *lru_gen_eviction(struct folio *folio)
 	return NULL;
 }
 
-static bool lru_gen_test_recent(void *shadow, bool file, struct lruvec **lruvec,
+static bool lru_gen_test_recent(void *shadow, struct lruvec **lruvec,
 				unsigned long *token, bool *workingset)
 {
 	return false;
@@ -432,8 +434,7 @@ bool workingset_test_recent(void *shadow, bool file, bool *workingset,
 		bool recent;
 
 		rcu_read_lock();
-		recent = lru_gen_test_recent(shadow, file, &eviction_lruvec,
-					     &eviction, workingset);
+		recent = lru_gen_test_recent(shadow, &eviction_lruvec, &eviction, workingset);
 		rcu_read_unlock();
 		return recent;
 	}
-- 
2.47.0.338.g60cca15819-goog