[PATCH v2] mm/mglru: fix a no-op lru_gen_clear_refs() call in lru_lazyfree()

["Hui Zhu" <hui.zhu@linux.dev>]

From: Hui Zhu <zhuhui@kylinos.cn>

lru_lazyfree() calls lru_gen_clear_refs() after lruvec_del_folio(), but
lru_gen_del_folio() has already cleared LRU_GEN_MASK, so folio_lru_gen()
returns -1 and lru_gen_clear_refs() exits at its "gen < 0" check without
clearing the refs or PG_workingset.

A MADV_FREE folio therefore keeps the state it had before madvise(2),
even though madvise_free_pte_range() has already cleared the accessed and
dirty bits of its PTEs.  lru_gen_folio_seq() places it one generation
younger, and PG_workingset puts it in the last tier, so sort_folio()
promotes and protects it instead of evicting it and charges it to
lrugen->protected[], which read_ctrl_pos() uses to balance anon against
file reclaim.  __folio_inc_gen() clears the refs but not PG_workingset,
so this repeats every time the folio reaches the oldest generation.  If
the folio is redirtied first, ttu_anon_lazyfree_folio() keeps it and
lru_gen_eviction() packs the stale state into the shadow entry, so a
later refault is promoted to the youngest generation.

Call lru_gen_clear_refs() while the folio is still on the generation
list, as deactivate_file_folio() and folio_deactivate() already do.  Its
return value only says whether the folio is in the oldest generation,
which lru_lazyfree() does not need because it re-adds the folio below.
Keep folio_clear_referenced() unconditional: the classic LRU needs it,
and it also covers the folios that lru_gen_clear_refs() skips.

Fixes: cc8ec7be78ff ("mm/mglru: optimize deactivation")
Suggested-by: Barry Song <baohua@kernel.org>
Signed-off-by: Hui Zhu <zhuhui@kylinos.cn>
---
Changelog:
v2:
Call lru_gen_clear_refs() before lruvec_del_folio() instead of
open-coding the flag clears; drop the lru_gen_enabled() branch and
the comment (Barry Song).

 mm/folio.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/mm/folio.c b/mm/folio.c
index 35e242b48870..ea3adb6a89d7 100644
--- a/mm/folio.c
+++ b/mm/folio.c
@@ -597,12 +597,10 @@ static void lru_lazyfree(struct lruvec *lruvec, struct folio *folio)
 	    folio_test_swapcache(folio) || folio_test_unevictable(folio))
 		return;
 
+	lru_gen_clear_refs(folio);
 	lruvec_del_folio(lruvec, folio);
 	folio_clear_active(folio);
-	if (lru_gen_enabled())
-		lru_gen_clear_refs(folio);
-	else
-		folio_clear_referenced(folio);
+	folio_clear_referenced(folio);
 	/*
 	 * Lazyfree folios are clean anonymous folios.  They have
 	 * the swapbacked flag cleared, to distinguish them from normal
-- 
2.53.0