mm: memcontrol: prepare for reparenting LRU pages for lruvec lock

The following diagram illustrates how to ensure the safety of the folio
lruvec lock when LRU folios undergo reparenting.

In the folio_lruvec_lock(folio) function:

    rcu_read_lock();
retry:
    lruvec = folio_lruvec(folio);
    /* There is a possibility of folio reparenting at this point. */
    spin_lock(&lruvec->lru_lock);
    if (unlikely(lruvec_memcg(lruvec) != folio_memcg(folio))) {
        /*
         * The wrong lruvec lock was acquired, and a retry is required.
         * This is because the folio resides on the parent memcg lruvec
         * list.
         */
        spin_unlock(&lruvec->lru_lock);
        goto retry;
    }

    /* Reaching here indicates that folio_memcg() is stable. */


In the memcg_reparent_objcgs(memcg) function:

    spin_lock(&lruvec->lru_lock);
    spin_lock(&lruvec_parent->lru_lock);
    /* Transfer folios from the lruvec list to the parent's. */
    spin_unlock(&lruvec_parent->lru_lock);
    spin_unlock(&lruvec->lru_lock);

After acquiring the lruvec lock, it is necessary to verify whether the
folio has been reparented.  If reparenting has occurred, the new lruvec
lock must be reacquired.  During the LRU folio reparenting process, the
lruvec lock will also be acquired (this will be implemented in a
subsequent patch).  Therefore, folio_memcg() remains unchanged while the
lruvec lock is held.

Given that lruvec_memcg(lruvec) is always equal to folio_memcg(folio)
after the lruvec lock is acquired, the lruvec_memcg_debug() check is
redundant.  Hence, it is removed.

This patch serves as a preparation for the reparenting of LRU folios.

Link: https://lore.kernel.org/23f22cbb1419f277a3483018b32158ae2b86c666.1772711148.git.zhengqi.arch@bytedance.com
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Allen Pais <apais@linux.microsoft.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Chen Ridong <chenridong@huawei.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Hamza Mahfooz <hamzamahfooz@linux.microsoft.com>
Cc: Harry Yoo <harry.yoo@oracle.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Imran Khan <imran.f.khan@oracle.com>
Cc: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam Howlett <Liam.Howlett@oracle.com>
Cc: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Usama Arif <usamaarif642@gmail.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Wei Xu <weixugc@google.com>
Cc: Yosry Ahmed <yosry@kernel.org>
Cc: Yuanchu Xie <yuanchu@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

2 files changed, 18 insertions, 19 deletions

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 38f94c7271c1..12982875073e 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -741,7 +741,15 @@ out:
  * folio_lruvec - return lruvec for isolating/putting an LRU folio
  * @folio: Pointer to the folio.
  *
- * This function relies on folio->mem_cgroup being stable.
+ * Call with rcu_read_lock() held to ensure the lifetime of the returned lruvec.
+ * Note that this alone will NOT guarantee the stability of the folio->lruvec
+ * association; the folio can be reparented to an ancestor if this races with
+ * cgroup deletion.
+ *
+ * Use folio_lruvec_lock() to ensure both lifetime and stability of the binding.
+ * Once a lruvec is locked, folio_lruvec() can be called on other folios, and
+ * their binding is stable if the returned lruvec matches the one the caller has
+ * locked. Useful for lock batching.
  */
 static inline struct lruvec *folio_lruvec(struct folio *folio)
 {
@@ -764,15 +772,6 @@ struct lruvec *folio_lruvec_lock_irq(struct folio *folio);
 struct lruvec *folio_lruvec_lock_irqsave(struct folio *folio,
 						unsigned long *flags);
 
-#ifdef CONFIG_DEBUG_VM
-void lruvec_memcg_debug(struct lruvec *lruvec, struct folio *folio);
-#else
-static inline
-void lruvec_memcg_debug(struct lruvec *lruvec, struct folio *folio)
-{
-}
-#endif
-
 static inline
 struct mem_cgroup *mem_cgroup_from_css(struct cgroup_subsys_state *css){
 	return css ? container_of(css, struct mem_cgroup, css) : NULL;
@@ -1198,11 +1197,6 @@ static inline struct lruvec *folio_lruvec(struct folio *folio)
 	return &pgdat->__lruvec;
 }
 
-static inline
-void lruvec_memcg_debug(struct lruvec *lruvec, struct folio *folio)
-{
-}
-
 static inline struct mem_cgroup *parent_mem_cgroup(struct mem_cgroup *memcg)
 {
 	return NULL;
@@ -1261,6 +1255,7 @@ static inline struct lruvec *folio_lruvec_lock(struct folio *folio)
 {
 	struct pglist_data *pgdat = folio_pgdat(folio);
 
+	rcu_read_lock();
 	spin_lock(&pgdat->__lruvec.lru_lock);
 	return &pgdat->__lruvec;
 }
@@ -1269,6 +1264,7 @@ static inline struct lruvec *folio_lruvec_lock_irq(struct folio *folio)
 {
 	struct pglist_data *pgdat = folio_pgdat(folio);
 
+	rcu_read_lock();
 	spin_lock_irq(&pgdat->__lruvec.lru_lock);
 	return &pgdat->__lruvec;
 }
@@ -1278,6 +1274,7 @@ static inline struct lruvec *folio_lruvec_lock_irqsave(struct folio *folio,
 {
 	struct pglist_data *pgdat = folio_pgdat(folio);
 
+	rcu_read_lock();
 	spin_lock_irqsave(&pgdat->__lruvec.lru_lock, *flagsp);
 	return &pgdat->__lruvec;
 }
@@ -1500,23 +1497,26 @@ static inline struct lruvec *parent_lruvec(struct lruvec *lruvec)
 
 static inline void lruvec_lock_irq(struct lruvec *lruvec)
 {
+	rcu_read_lock();
 	spin_lock_irq(&lruvec->lru_lock);
 }
 
 static inline void lruvec_unlock(struct lruvec *lruvec)
 {
 	spin_unlock(&lruvec->lru_lock);
+	rcu_read_unlock();
 }
 
 static inline void lruvec_unlock_irq(struct lruvec *lruvec)
 {
 	spin_unlock_irq(&lruvec->lru_lock);
+	rcu_read_unlock();
 }
 
-static inline void lruvec_unlock_irqrestore(struct lruvec *lruvec,
-		unsigned long flags)
+static inline void lruvec_unlock_irqrestore(struct lruvec *lruvec, unsigned long flags)
 {
 	spin_unlock_irqrestore(&lruvec->lru_lock, flags);
+	rcu_read_unlock();
 }
 
 /* Test requires a stable folio->memcg binding, see folio_memcg() */
diff --git a/include/linux/swap.h b/include/linux/swap.h
index 4b1f13b5bbad..ea08e2afa2b4 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -310,8 +310,7 @@ extern unsigned long totalreserve_pages;
 
 /* linux/mm/swap.c */
 void lru_note_cost_unlock_irq(struct lruvec *lruvec, bool file,
-		unsigned int nr_io, unsigned int nr_rotated)
-		__releases(lruvec->lru_lock);
+		unsigned int nr_io, unsigned int nr_rotated);
 void lru_note_cost_refault(struct folio *);
 void folio_add_lru(struct folio *);
 void folio_add_lru_vma(struct folio *, struct vm_area_struct *);