mm: mglru: fix stale batch updates after memcg reparenting

The mglru page table walker batches per-generation size deltas in
walk->nr_pages while walking page tables without holding the lruvec lock.
The reset_batch_size() later folds those deltas into walk->lruvec under
the lruvec lock.

The page table walker can run concurrently with the memcg reparenting path
as follows:

CPU0                           CPU1
====                           ====

walk_mm
--> walk_page_range
    --> update_batch_size
        --> walk->nr_pages += delta

                              mem_cgroup_css_offline
                              --> memcg_reparent_objcgs
                                  --> lock lruvec
                                      lru_gen_reparent_memcg
                                      --> reparent child folios to parent
                                      unlock lruvec

    lock lruvec
    reset_batch_size
    --> child lrugen->nr_pages += delta

This will trigger the following warning in lru_gen_exit_memcg():

	VM_WARN_ON_ONCE(memchr_inv(lruvec->lrugen.nr_pages, 0,
				   sizeof(lruvec->lrugen.nr_pages)));

And the user-visible impact of underestimated nr_pages in MGLRU was
premature OOMs because MGLRU does not try to reclaim memory when nr_pages
reaches zero, but there are still more pages.

To fix it, make reset_batch_size() check CSS_DYING under RCU before
flushing the pending batch.  A non-dying memcg keeps the original lruvec
stable against RCU-delayed offlining; a dying memcg redirects the deltas
to the first non-dying ancestor.

Link: https://lore.kernel.org/20260710154318.75388-1-qi.zheng@linux.dev
Fixes: f304652609ea ("mm: vmscan: prepare for reparenting MGLRU folios")
Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Reported-by: Peiyang He <peiyang_he@smail.nju.edu.cn>
Closes: https://lore.kernel.org/all/5A9E929D82717101+12fcf643-efb8-4b9a-a53a-1e28cc894f0b@smail.nju.edu.cn
Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Wei Xu <weixugc@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index e1f46a0016fcfd0269552795ca47ef696f59ab1f..95726067767815b8cd175dda725da40d2f4e2303 100644 (file)
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -1472,6 +1472,31 @@ static inline void lruvec_lock_irq(struct lruvec *lruvec)
        spin_lock_irq(&lruvec->lru_lock);
 }
 
+static inline struct lruvec *lruvec_live_lock_irq(struct lruvec *lruvec)
+{
+#ifdef CONFIG_MEMCG
+       struct pglist_data *pgdat = lruvec_pgdat(lruvec);
+       struct mem_cgroup *memcg = lruvec_memcg(lruvec);
+
+       rcu_read_lock();
+
+       /*
+        * The memcg can be NULL when the memory controller is disabled.
+        * Otherwise, the caller keeps the memcg owning @lruvec alive.
+        */
+       while (unlikely(memcg && css_is_dying(&memcg->css))) {
+               memcg = parent_mem_cgroup(memcg);
+               lruvec = mem_cgroup_lruvec(memcg, pgdat);
+       }
+
+       spin_lock_irq(&lruvec->lru_lock);
+#else
+       lruvec_lock_irq(lruvec);
+#endif
+
+       return lruvec;
+}
+
 static inline void lruvec_unlock(struct lruvec *lruvec)
 {
        spin_unlock(&lruvec->lru_lock);

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 35c3bb15ae96a395a09afe863bd55aa64ec12ea1..1a142c58700d00de7a443792a5e3457f5f0cb56c 100644 (file)
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3265,7 +3265,7 @@ static void update_batch_size(struct lru_gen_mm_walk *walk, struct folio *folio,
 static void reset_batch_size(struct lru_gen_mm_walk *walk)
 {
        int gen, type, zone;
-       struct lruvec *lruvec = walk->lruvec;
+       struct lruvec *lruvec = lruvec_live_lock_irq(walk->lruvec);
        struct lru_gen_folio *lrugen = &lruvec->lrugen;
 
        walk->batched = 0;
@@ -3285,6 +3285,8 @@ static void reset_batch_size(struct lru_gen_mm_walk *walk)
                        lru += LRU_ACTIVE;
                __update_lru_size(lruvec, lru, zone, delta);
        }
+
+       lruvec_unlock_irq(lruvec);
 }
 
 static int should_skip_vma(unsigned long start, unsigned long end, struct mm_walk *args)
@@ -3779,11 +3781,8 @@ static void walk_mm(struct mm_struct *mm, struct lru_gen_mm_walk *walk)
                        mmap_read_unlock(mm);
                }
 
-               if (walk->batched) {
-                       lruvec_lock_irq(lruvec);
+               if (walk->batched)
                        reset_batch_size(walk);
-                       lruvec_unlock_irq(lruvec);
-               }
 
                cond_resched();
        } while (err == -EAGAIN);
@@ -4867,9 +4866,7 @@ retry:
        walk = current->reclaim_state->mm_walk;
        if (walk && walk->batched) {
                walk->lruvec = lruvec;
-               lruvec_lock_irq(lruvec);
                reset_batch_size(walk);
-               lruvec_unlock_irq(lruvec);
        }
 
        mod_lruvec_state(lruvec, PGDEMOTE_KSWAPD + reclaimer_offset(sc),