[PATCH 7/7] mm/mglru: improve code readability and harden folio_inc_gen

From: Kairui Song <[email protected]>

The helper should never be called for an off-list folio, and it always
expects the folio to be in the oldest generation before doing any
cmpxchg. Add a sanity check for the off-list case: if it is ever
violated, bail out and keep the folio flags untouched to minimize the
damage, instead of silently treating the folio as if it were in the
oldest generation and promoting it updating the flags to an unexpected
status.

Also rename the variables to clearly distinguish the folio's current
gen from the oldest gen.

Signed-off-by: Kairui Song <[email protected]>
---
 mm/vmscan.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 7169cac60869..7e3ae0c6cba3 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3308,18 +3308,22 @@ static int folio_inc_gen(struct lruvec *lruvec, struct folio *folio)
 {
 	int type = folio_is_file_lru(folio);
 	struct lru_gen_folio *lrugen = &lruvec->lrugen;
-	int new_gen, old_gen = lru_gen_from_seq(lrugen->min_seq[type]);
+	int new_gen, old_gen, min_gen = lru_gen_from_seq(lrugen->min_seq[type]);
 	unsigned long new_flags, old_flags = READ_ONCE(*folio_flags(folio, 0));
 
 	do {
-		new_gen = lru_gen_from_flags(old_flags);
+		old_gen = lru_gen_from_flags(old_flags);
+		/* This helper should never be called for off-list folios */
+		VM_WARN_ON_ONCE(old_gen < 0);
+		if (old_gen < 0)
+			return min_gen;
 
 		/* folio_update_gen() has promoted this page? */
-		if (new_gen >= 0 && new_gen != old_gen)
-			return new_gen;
+		if (old_gen != min_gen)
+			return old_gen;
 
 		new_flags = old_flags;
-		new_gen = (old_gen + 1) % MAX_NR_GENS;
+		new_gen = (min_gen + 1) % MAX_NR_GENS;
 		lru_gen_set_flags(&new_flags, new_gen);
 		lru_refs_set_flags(&new_flags, 0);
 	} while (!try_cmpxchg(folio_flags(folio, 0), &old_flags, new_flags));

-- 
2.55.0