Since the deferred split queue became a list_lru, the isolate callback moves the folios it took a reference on to an on-stack list, linked through folio->_deferred_list, and deferred_split_scan() takes them off that list with list_del_init() without holding the list_lru lock. deferred_split_folio() does not know about the on-stack list. It only holds the list_lru lock when it looks at the same list_head, and it can run for such a folio: a THP is queued when it is mapped, as a candidate for the underused check, and is queued "again" when a part of it gets unmapped: CPU0 deferred_split_scan() CPU1 deferred_split_folio() list_del_init(&folio->_deferred_list) next->prev = prev prev->next = next entry->next = entry list_lru_lock_irqsave() folio_set_partially_mapped() __list_lru_add() list_empty(item) -> true list_add_tail(item, &l->list) item->next = &l->list item->prev = tail entry->prev = entry The folio now sits on the list_lru with ->prev pointing to itself. The next removal leaves the neighbours pointing to it, and they still do so after the folio has been freed. With CONFIG_DEBUG_LIST: list_del corruption. prev->next should be ffd4000000270090, but was ffd4000000278090. (prev=ffd4000000270090) Fix it by grabbing the lock around list_del_init() so deferred_split_folio() cannot race with it. Fixes: fafaeceb89a5 ("mm: switch deferred split shrinker to list_lru") Cc: stable@vger.kernel.org Reported-by: Josef Bacik Reviewed-by: Johannes Weiner Reviewed-by: Mani Tofigh Tested-by: Mani Tofigh Reviewed-by: Lance Yang Tested-by: Muhammad Usama Anjum Signed-off-by: David Hildenbrand (Arm) --- mm/huge_memory.c | 15 +++++++++++++-- 1 file changed, 13 insertions(+), 2 deletions(-) diff --git a/mm/huge_memory.c b/mm/huge_memory.c index bd44d58e86845..4c0194af9016d 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -4522,8 +4522,7 @@ int split_folio_to_list(struct folio *folio, struct list_head *list) * * It is unsafe to call folio_unqueue_deferred_split() until folio refcount is * zero: because even when the list_lru lock is held, a non-empty - * _deferred_list might be in use on deferred_split_scan()'s unlocked - * on-stack list. + * _deferred_list might be in use on deferred_split_scan()'s on-stack list. * * The list_lru sublist is determined by folio's memcg: it is therefore * important to unqueue deferred split before changing folio memcg. @@ -4665,10 +4664,22 @@ static unsigned long deferred_split_scan(struct shrinker *shrink, deferred_split_isolate, &dispose); list_for_each_entry_safe(folio, next, &dispose, _deferred_list) { + struct mem_cgroup *memcg; + struct list_lru_one *lru; bool did_split = false; bool underused = false; + /* + * Sync with deferred_split_folio(), which can requeue the + * folio once list_empty() is true, and sets PG_partially_mapped. + */ + rcu_read_lock(); + memcg = folio_memcg(folio); + lru = list_lru_lock_irq(&deferred_split_lru, folio_nid(folio), + &memcg); list_del_init(&folio->_deferred_list); + list_lru_unlock_irq(lru); + rcu_read_unlock(); if (!folio_test_partially_mapped(folio)) { /* -- 2.43.0