Re: [PATCH] mm: memcontrol: fix swap counter leak from offline cgroup

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Mar 06, 2025 at 10:54:12AM +0800, Kairui Song wrote:
> On Thu, Mar 6, 2025 at 10:32 AM Muchun Song <songmuchun@bytedance.com> wrote:
> >
> > The commit 6769183166b3 has removed the parameter of id from
> > swap_cgroup_record() and get the memcg id from
> > mem_cgroup_id(folio_memcg(folio)). However, the caller of it
> > may update a different memcg's counter instead of
> > folio_memcg(folio). E.g. in the caller of mem_cgroup_swapout(),
> > @swap_memcg could be different with @memcg and update the counter
> > of @swap_memcg, but swap_cgroup_record() records the wrong memcg's
> > ID. When it is uncharged from __mem_cgroup_uncharge_swap(), the
> > swap counter will leak since the wrong recorded ID. Fix it by
> > bring the parameter of id back.
> >
> > Fixes: 6769183166b3 ("mm/swap_cgroup: decouple swap cgroup recording and clearing")
> > Cc: <stable@vger.kernel.org>
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Looking at the original commit again, we also should do this:

---

From 2685ca87d73d0c2b91cfd6959e381a40db235119 Mon Sep 17 00:00:00 2001
From: Johannes Weiner <hannes@cmpxchg.org>
Date: Thu, 6 Mar 2025 09:31:42 -0500
Subject: [PATCH] mm: swap_cgroup: remove double initialization of locals

Fixes: 6769183166b3 ("mm/swap_cgroup: decouple swap cgroup recording and clearing")
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/swap_cgroup.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/mm/swap_cgroup.c b/mm/swap_cgroup.c
index 1007c30f12e2..de779fed8c21 100644
--- a/mm/swap_cgroup.c
+++ b/mm/swap_cgroup.c
@@ -92,8 +92,7 @@ void swap_cgroup_record(struct folio *folio, unsigned short id,
  */
 unsigned short swap_cgroup_clear(swp_entry_t ent, unsigned int nr_ents)
 {
-	pgoff_t offset = swp_offset(ent);
-	pgoff_t end = offset + nr_ents;
+	pgoff_t offset, end;
 	struct swap_cgroup *map;
 	unsigned short old, iter = 0;
 
-- 
2.48.1