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mm: mglru: fix stale batch updates after memcg reparenting

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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: f304652609 ("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>
This commit is contained in:
Qi Zheng 2026-07-10 23:43:18 +08:00 committed by Andrew Morton
parent e3a0127eee
commit de4660898b
2 changed files with 29 additions and 7 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

25
include/linux/memcontrol.h
View File

@ -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);

11
mm/vmscan.c
View File

@ -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 @@ static int evict_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
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),