Re: [PATCH] mm/mglru: Use folio_mark_accessed to replace folio_set_active in PF

["Barry Song (Xiaomi)" <baohua@kernel.org>]

On Tue, Apr 28, 2026 at 2:23 AM Axel Rasmussen <axelrasmussen@google.com> wrote:
>
> For what it's worth, I agree with this change in principle.
>
> In production we set fault_around_bytes to 4096. That setting is
> surprisingly load-bearing (i.e. if I change it, even at a small
> experimental scale, I expect workloads to notice and complain). So I
> don't think I have an easy way to test this change under production
> workloads.
>
> Like Andrew said the workload in the commit message doesn't seem
> unreasonable, and the benefit is large.
>
> I guess the workload that would see a downside from this is one that
> heavily uses readahead pages but also generates many "one-time-use"
> pages instead of maintaining a "fixed" working set. Without activating
> the readahead pages, does it lose some of the readahead benefit
> because they are pushed out?
>
> About the Sashiko comments, the tier bits being cleared doesn't seem
> that problematic to me. However, the WORKINGSET_ACTIVATE counter issue
> seems worth fixing.
>

I am considering something more reasonable than simply
"fixing" the counter. Right now, MGLRU unconditionally
treats PF folios as WORKINGSET_ACTIVATE_BASE and neglects
other folios entirely. I am thinking of a better approach
that detects true recency. In the active/inactive case,
this is refault_distance < workingset_size.

In MGLRU, we might detect whether reclamation occurred
within the most recent one or two generations. I am
queuing the following for testing:

diff --git a/mm/workingset.c b/mm/workingset.c
index 07e6836d0502..8b552b3d7e37 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -271,10 +271,11 @@ static void *lru_gen_eviction(struct folio *folio)
  * Fills in @lruvec, @token, @workingset with the values unpacked from shadow.
  */
 static bool lru_gen_test_recent(void *shadow, struct lruvec **lruvec,
-				unsigned long *token, bool *workingset, bool file)
+				unsigned long *token, bool *workingset, bool file,
+				unsigned long *gen_distance)
 {
 	int memcg_id;
-	unsigned long max_seq;
+	unsigned long max_seq, distance;
 	struct mem_cgroup *memcg;
 	struct pglist_data *pgdat;
 
@@ -286,7 +287,10 @@ static bool lru_gen_test_recent(void *shadow, struct lruvec **lruvec,
 	max_seq = READ_ONCE((*lruvec)->lrugen.max_seq);
 	max_seq &= (file ? EVICTION_MASK : EVICTION_MASK_ANON) >> LRU_REFS_WIDTH;
 
-	return abs_diff(max_seq, *token >> LRU_REFS_WIDTH) < MAX_NR_GENS;
+	distance = abs_diff(max_seq, *token >> LRU_REFS_WIDTH);
+	if (gen_distance)
+		*gen_distance = distance;
+	return distance < MAX_NR_GENS;
 }
 
 static void lru_gen_refault(struct folio *folio, void *shadow)
@@ -294,7 +298,7 @@ static void lru_gen_refault(struct folio *folio, void *shadow)
 	bool recent;
 	int hist, tier, refs;
 	bool workingset;
-	unsigned long token;
+	unsigned long token, distance;
 	struct lruvec *lruvec;
 	struct lru_gen_folio *lrugen;
 	int type = folio_is_file_lru(folio);
@@ -302,7 +306,8 @@ static void lru_gen_refault(struct folio *folio, void *shadow)
 
 	rcu_read_lock();
 
-	recent = lru_gen_test_recent(shadow, &lruvec, &token, &workingset, type);
+	recent = lru_gen_test_recent(shadow, &lruvec, &token, &workingset, type,
+				     &distance);
 	if (lruvec != folio_lruvec(folio))
 		goto unlock;
 
@@ -319,9 +324,11 @@ static void lru_gen_refault(struct folio *folio, void *shadow)
 
 	atomic_long_add(delta, &lrugen->refaulted[hist][type][tier]);
 
-	/* see folio_add_lru() where folio_set_active() will be called */
-	if (lru_gen_in_fault())
+	/* If the folio was reclaimed very recently. */
+	if (distance <= MIN_LRU_GENS) {
+		folio_set_active(folio);
 		mod_lruvec_state(lruvec, WORKINGSET_ACTIVATE_BASE + type, delta);
+	}
 
 	if (workingset) {
 		folio_set_workingset(folio);
@@ -442,7 +449,7 @@ bool workingset_test_recent(void *shadow, bool file, bool *workingset,
 
 		rcu_read_lock();
 		recent = lru_gen_test_recent(shadow, &eviction_lruvec, &eviction,
-					     workingset, file);
+					     workingset, file, NULL);
 		rcu_read_unlock();
 		return recent;
 	}