mm: multi-gen LRU: update lrugen->nr_pages during lruvec reparenting

lruvec_reparent_relocate() does not update lrugen->nr_pages field on
reparenting, leading to a DEBUG_VM warning due to non-zero nr_pages at
memory cgroup exit:

  WARNING: CPU: 1 PID: 237 at mm/vmscan.c:5622 lru_gen_exit_memcg+0x13a/0x190
  [...]
  RIP: 0010:lru_gen_exit_memcg+0x13a/0x190
  [...]
  Call Trace:
   <TASK>
   mem_cgroup_css_free+0x83/0xd0
   css_free_rwork_fn+0x4e/0x3b0
   process_one_work+0x222/0x640
   worker_thread+0x183/0x330
   ? __pfx_worker_thread+0x10/0x10
   kthread+0xd6/0x110
   ? __pfx_kthread+0x10/0x10
   ret_from_fork+0x34/0x50
   ? __pfx_kthread+0x10/0x10
   ret_from_fork_asm+0x1a/0x30
  </TASK>

lru_gen_update_size() cannot be used here because we need to update
the counters for multiple folios. Introduce a new helper function
lru_gen_lruvec_update_size() and call it during lruvec reparenting.

Orabug: 37997618

Reviewed-by: Imran Khan <[email protected]>
Signed-off-by: Harry Yoo <[email protected]>

Author: Harry Yoo

include/linux/mm_inline.h

@@ -172,6 +172,15 @@ static inline bool lru_gen_is_active(struct lruvec *lruvec, int gen)
 	return gen == lru_gen_from_seq(max_seq) || gen == lru_gen_from_seq(max_seq - 1);
 }
 
+static inline void lru_gen_lruvec_update_size(struct lruvec *lruvec, int gen,
+					      int type, int zone, long delta)
+{
+	struct lru_gen_folio *lrugen = &lruvec->lrugen;
+
+	WRITE_ONCE(lrugen->nr_pages[gen][type][zone],
+		   lrugen->nr_pages[gen][type][zone] + delta);
+}
+
 static inline void lru_gen_update_size(struct lruvec *lruvec, struct folio *folio,
 				       int old_gen, int new_gen)
 {

mm/memcontrol.c

@@ -271,6 +271,10 @@ static void lruvec_reparent_relocate(struct mem_cgroup *src, struct mem_cgroup *
 		for_each_gen_type_zone(gen, type, zone) {
 			list_splice_tail_init(&src_lrugen->folios[gen][type][zone],
 					      &dst_lrugen->folios[gen][type][zone]);
+
+			long delta = src_lrugen->nr_pages[gen][type][zone];
+			lru_gen_lruvec_update_size(src_lruvec, gen, type, zone, -delta);
+			lru_gen_lruvec_update_size(dst_lruvec, gen, type, zone, delta);
 		}
 
 		mz_src = container_of(src_lruvec, struct mem_cgroup_per_node, lruvec);