[PATCH v1 11/26] mm: page_io: prevent memory cgroup release in page_io module

Qi Zheng
2025-10-28T14:03:24Z

From: Muchun Song <songmuchun@bytedance.com>

In the near future, a folio will no longer pin its corresponding
memory cgroup. To ensure safety, it will only be appropriate to
hold the rcu read lock or acquire a reference to the memory cgroup
returned by folio_memcg(), thereby preventing it from being released.

In the current patch, the rcu read lock is employed to safeguard
against the release of the memory cgroup in swap_writepage() and
bio_associate_blkg_from_page().

This serves as a preparatory measure for the reparenting of the
LRU pages.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
---
mm/page_io.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/mm/page_io.c b/mm/page_io.c
index 3c342db77ce38..ec7720762042c 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -276,10 +276,14 @@ int swap_writeout(struct folio *folio, struct swap_iocb **swap_plug)
count_mthp_stat(folio_order(folio), MTHP_STAT_ZSWPOUT);
goto out_unlock;
}
+
+ rcu_read_lock();
if (!mem_cgroup_zswap_writeback_enabled(folio_memcg(folio))) {
+ rcu_read_unlock();
folio_mark_dirty(folio);
return AOP_WRITEPAGE_ACTIVATE;
}
+ rcu_read_unlock();

__swap_writepage(folio, swap_plug);
return 0;
@@ -307,11 +311,11 @@ static void bio_associate_blkg_from_page(struct bio *bio, struct folio *folio)
struct cgroup_subsys_state *css;
struct mem_cgroup *memcg;

- memcg = folio_memcg(folio);
- if (!memcg)
+ if (!folio_memcg_charged(folio))
return;

rcu_read_lock();
+ memcg = folio_memcg(folio);
css = cgroup_e_css(memcg->css.cgroup, &io_cgrp_subsys);
bio_associate_blkg_from_css(bio, css);
rcu_read_unlock();
--
2.20.1