Multi-gen LRU: fix workingset accounting

On Android app cycle workloads, MGLRU showed a significant reduction in
workingset refaults although pgpgin/pswpin remained relatively unchanged. 
This indicated MGLRU may be undercounting workingset refaults.

This has impact on userspace programs, like Android's LMKD, that monitor
workingset refault statistics to detect thrashing.

It was found that refaults were only accounted if the MGLRU shadow entry
was for a recently evicted folio.  However, recently evicted folios should
be accounted as workingset activation, and refaults should be accounted
regardless of recency.

Fix MGLRU's workingset refault and activation accounting to more closely
match that of the conventional active/inactive LRU.

Link: https://lkml.kernel.org/r/20230523205922.3852731-1-kaleshsingh@google.com
Fixes: ac35a490 ("mm: multi-gen LRU: minimal implementation")
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
Reported-by: Charan Teja Kalla <quic_charante@quicinc.com>
Acked-by: Yu Zhao <yuzhao@google.com>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Jan Alexander Steffens (heftig) <heftig@archlinux.org>
Cc: Oleksandr Natalenko <oleksandr@natalenko.name>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/vmscan.c

@@ -4925,7 +4925,6 @@ static bool sort_folio(struct lruvec *lruvec, struct folio *folio, int tier_idx)
 
 		WRITE_ONCE(lrugen->protected[hist][type][tier - 1],
 			   lrugen->protected[hist][type][tier - 1] + delta);
-		__mod_lruvec_state(lruvec, WORKINGSET_ACTIVATE_BASE + type, delta);
 		return true;
 	}
 

mm/workingset.c

@@ -278,6 +278,7 @@ static bool lru_gen_test_recent(void *shadow, bool file, struct lruvec **lruvec,
 
 static void lru_gen_refault(struct folio *folio, void *shadow)
 {
+	bool recent;
 	int hist, tier, refs;
 	bool workingset;
 	unsigned long token;
@@ -288,10 +289,13 @@ static void lru_gen_refault(struct folio *folio, void *shadow)
 
 	rcu_read_lock();
 
-	if (!lru_gen_test_recent(shadow, type, &lruvec, &token, &workingset))
+	recent = lru_gen_test_recent(shadow, type, &lruvec, &token, &workingset);
+	if (lruvec != folio_lruvec(folio))
 		goto unlock;
 
-	if (lruvec != folio_lruvec(folio))
+	mod_lruvec_state(lruvec, WORKINGSET_REFAULT_BASE + type, delta);
+
+	if (!recent)
 		goto unlock;
 
 	lrugen = &lruvec->lrugen;
@@ -302,7 +306,7 @@ static void lru_gen_refault(struct folio *folio, void *shadow)
 	tier = lru_tier_from_refs(refs);
 
 	atomic_long_add(delta, &lrugen->refaulted[hist][type][tier]);
-	mod_lruvec_state(lruvec, WORKINGSET_REFAULT_BASE + type, delta);
+	mod_lruvec_state(lruvec, WORKINGSET_ACTIVATE_BASE + type, delta);
 
 	/*
 	 * Count the following two cases as stalls: