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From: Qi Zheng In-Reply-To: <4a1b69d2-df29-4204-91fd-bb00b52350db@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Stat-Signature: ika9og6abej1jk3qjjzypm44rd5yr4yp X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 7E8D74000D X-HE-Tag: 1768557044-330801 X-HE-Meta: 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 xYvSVnF7 EJm2NjZ1JJR21qXSv8LgQHCFU1izhBDMRT0/Hb4p5w5a219fDNqum2wvlyWUH28sNVIwFC31lCAaIfg3QpD6fUs0YPODbMcmCoTpueYOCRYlMTK7qcmd6WQ7shciPk1wnEg9yKzTeZyycgE9+VrvIwklExGJ6APPvCl2BjHjMCep40zHMIol1fP7OFHr72/WMZmT+CSyxt1NgOJtvyAYPDkZ/sVPLkXWP3oyln+hy+rNCjpd+4aSsYK/SspUT9xI517tygt+UpR5t9sBVc6GxCH2Sn5Hw/pU5X/UUa3u8IGw5RI/pglSpqSGpYdP7KmOf64fg X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 1/16/26 5:43 PM, Muchun Song wrote: > > > On 2026/1/14 19:32, Qi Zheng wrote: >> From: Muchun Song >> >> 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. >> >> Signed-off-by: Muchun Song >> Signed-off-by: Qi Zheng >> Acked-by: Johannes Weiner >> --- >>   include/linux/memcontrol.h | 45 +++++++++++++++++++---------- >>   include/linux/swap.h       |  1 + >>   mm/compaction.c            | 29 +++++++++++++++---- >>   mm/memcontrol.c            | 59 +++++++++++++++++++++----------------- >>   mm/swap.c                  |  4 +++ >>   5 files changed, 91 insertions(+), 47 deletions(-) >> >> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h >> index 4b6f20dc694ba..26c3c0e375f58 100644 >> --- a/include/linux/memcontrol.h >> +++ b/include/linux/memcontrol.h >> @@ -742,7 +742,15 @@ static inline struct lruvec >> *mem_cgroup_lruvec(struct mem_cgroup *memcg, >>    * 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) >>   { >> @@ -761,18 +769,15 @@ struct mem_cgroup >> *get_mem_cgroup_from_current(void); >>   struct mem_cgroup *get_mem_cgroup_from_folio(struct folio *folio); >>   struct lruvec *folio_lruvec_lock(struct folio *folio); >> +    __acquires(&lruvec->lru_lock) >> +    __acquires(rcu) >>   struct lruvec *folio_lruvec_lock_irq(struct folio *folio); >> +    __acquires(&lruvec->lru_lock) >> +    __acquires(rcu) >>   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 >> +    __acquires(&lruvec->lru_lock) >> +    __acquires(rcu) >>   static inline >>   struct mem_cgroup *mem_cgroup_from_css(struct cgroup_subsys_state >> *css){ >> @@ -1199,11 +1204,6 @@ 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; >> @@ -1262,6 +1262,7 @@ 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; >>   } >> @@ -1270,6 +1271,7 @@ 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; >>   } >> @@ -1279,6 +1281,7 @@ 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; >>   } >> @@ -1500,24 +1503,36 @@ static inline struct lruvec >> *parent_lruvec(struct lruvec *lruvec) >>   } >>   static inline void lruvec_lock_irq(struct lruvec *lruvec) >> +    __acquires(&lruvec->lru_lock) >> +    __acquires(rcu) > > It seems that functions marked as `inline` cannot be decorated with > `__acquires`? We’ve had to move these little helpers into `memcontrol.c` > and declare them as extern, but they’re so short that it hardly feels Right, I received a compilation error reported LKP: All errors (new ones prefixed by >>): In file included from crypto/ahash.c:26: In file included from include/net/netlink.h:6: In file included from include/linux/netlink.h:9: In file included from include/net/scm.h:9: In file included from include/linux/security.h:35: In file included from include/linux/bpf.h:32: >> include/linux/memcontrol.h:772:14: error: use of undeclared identifier 'lruvec' 772 | __acquires(&lruvec->lru_lock) | ^~~~~~ include/linux/memcontrol.h:773:13: error: use of undeclared identifier 'rcu' 773 | __acquires(rcu) | ^~~ include/linux/memcontrol.h:775:14: error: use of undeclared identifier 'lruvec' 775 | __acquires(&lruvec->lru_lock) | ^~~~~~ include/linux/memcontrol.h:776:13: error: use of undeclared identifier 'rcu' 776 | __acquires(rcu) | ^~~ include/linux/memcontrol.h:779:14: error: use of undeclared identifier 'lruvec' 779 | __acquires(&lruvec->lru_lock) | ^~~~~~ include/linux/memcontrol.h:780:13: error: use of undeclared identifier 'rcu' 780 | __acquires(rcu) | ^~~ include/linux/memcontrol.h:1507:13: error: use of undeclared identifier 'rcu' 1507 | __acquires(rcu) | ^~~ include/linux/memcontrol.h:1515:13: error: use of undeclared identifier 'rcu' 1515 | __releases(rcu) | ^~~ include/linux/memcontrol.h:1523:13: error: use of undeclared identifier 'rcu' 1523 | __releases(rcu) | ^~~ include/linux/memcontrol.h:1532:13: error: use of undeclared identifier 'rcu' 1532 | __releases(rcu) And I reproduced this error with the following configuration: 1. enable CONFIG_WARN_CONTEXT_ANALYSIS_ALL 2. make CC=clang bzImage (clang version >= 22) > worth the trouble. My own inclination is to drop the `__acquires` > annotations—mainly for performance reasons. If no one else objects, I will drop __acquires/__releases in the next version. > >>   { >> +    rcu_read_lock(); >>       spin_lock_irq(&lruvec->lru_lock); >>   } >>   static inline void lruvec_unlock(struct lruvec *lruvec) >> +    __releases(&lruvec->lru_lock) >> +    __releases(rcu) >>   { >>       spin_unlock(&lruvec->lru_lock); >> +    rcu_read_unlock(); >>   } >>   static inline void lruvec_unlock_irq(struct lruvec *lruvec) >> +    __releases(&lruvec->lru_lock) >> +    __releases(rcu) >>   { >>       spin_unlock_irq(&lruvec->lru_lock); >> +    rcu_read_unlock(); >>   } >>   static inline void lruvec_unlock_irqrestore(struct lruvec *lruvec, >>           unsigned long flags) >> +    __releases(&lruvec->lru_lock) >> +    __releases(rcu) >>   { >>       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 62fc7499b4089..e60f45b48e74d 100644 >> --- a/include/linux/swap.h >> +++ b/include/linux/swap.h >> @@ -330,6 +330,7 @@ extern unsigned long totalreserve_pages; >>   void lru_note_cost_unlock_irq(struct lruvec *lruvec, bool file, >>           unsigned int nr_io, unsigned int nr_rotated) >>           __releases(lruvec->lru_lock); >> +        __releases(rcu) > > Missed a semicolon. > >>   void lru_note_cost_refault(struct folio *); >>   void folio_add_lru(struct folio *); >>   void folio_add_lru_vma(struct folio *, struct vm_area_struct *); >> diff --git a/mm/compaction.c b/mm/compaction.c >> index c3e338aaa0ffb..3648ce22c8072 100644 >> --- a/mm/compaction.c >> +++ b/mm/compaction.c >> @@ -518,6 +518,24 @@ static bool compact_lock_irqsave(spinlock_t >> *lock, unsigned long *flags, >>       return true; >>   } >> +static struct lruvec * >> +compact_folio_lruvec_lock_irqsave(struct folio *folio, unsigned long >> *flags, >> +                  struct compact_control *cc) >> +{ >> +    struct lruvec *lruvec; >> + >> +    rcu_read_lock(); >> +retry: >> +    lruvec = folio_lruvec(folio); >> +    compact_lock_irqsave(&lruvec->lru_lock, flags, cc); >> +    if (unlikely(lruvec_memcg(lruvec) != folio_memcg(folio))) { >> +        spin_unlock_irqrestore(&lruvec->lru_lock, *flags); >> +        goto retry; >> +    } >> + >> +    return lruvec; >> +} >> + >>   /* >>    * Compaction requires the taking of some coarse locks that are >> potentially >>    * very heavily contended. The lock should be periodically unlocked >> to avoid >> @@ -839,7 +857,7 @@ isolate_migratepages_block(struct compact_control >> *cc, unsigned long low_pfn, >>   { >>       pg_data_t *pgdat = cc->zone->zone_pgdat; >>       unsigned long nr_scanned = 0, nr_isolated = 0; >> -    struct lruvec *lruvec; >> +    struct lruvec *lruvec = NULL; >>       unsigned long flags = 0; >>       struct lruvec *locked = NULL; >>       struct folio *folio = NULL; >> @@ -1153,18 +1171,17 @@ isolate_migratepages_block(struct >> compact_control *cc, unsigned long low_pfn, >>           if (!folio_test_clear_lru(folio)) >>               goto isolate_fail_put; >> -        lruvec = folio_lruvec(folio); >> +        if (locked) >> +            lruvec = folio_lruvec(folio); >>           /* If we already hold the lock, we can skip some rechecking */ >> -        if (lruvec != locked) { >> +        if (lruvec != locked || !locked) { >>               if (locked) >>                   lruvec_unlock_irqrestore(locked, flags); >> -            compact_lock_irqsave(&lruvec->lru_lock, &flags, cc); >> +            lruvec = compact_folio_lruvec_lock_irqsave(folio, &flags, >> cc); >>               locked = lruvec; >> -            lruvec_memcg_debug(lruvec, folio); >> - >>               /* >>                * Try get exclusive access under lock. If marked for >>                * skip, the scan is aborted unless the current context >> diff --git a/mm/memcontrol.c b/mm/memcontrol.c >> index 548e67dbf2386..a1573600d4188 100644 >> --- a/mm/memcontrol.c >> +++ b/mm/memcontrol.c >> @@ -1201,23 +1201,6 @@ void mem_cgroup_scan_tasks(struct mem_cgroup >> *memcg, >>       } >>   } >> -#ifdef CONFIG_DEBUG_VM >> -void lruvec_memcg_debug(struct lruvec *lruvec, struct folio *folio) >> -{ >> -    struct mem_cgroup *memcg; >> - >> -    if (mem_cgroup_disabled()) >> -        return; >> - >> -    memcg = folio_memcg(folio); >> - >> -    if (!memcg) >> -        VM_BUG_ON_FOLIO(!mem_cgroup_is_root(lruvec_memcg(lruvec)), >> folio); >> -    else >> -        VM_BUG_ON_FOLIO(lruvec_memcg(lruvec) != memcg, folio); >> -} >> -#endif >> - >>   /** >>    * folio_lruvec_lock - Lock the lruvec for a folio. >>    * @folio: Pointer to the folio. >> @@ -1227,14 +1210,22 @@ void lruvec_memcg_debug(struct lruvec *lruvec, >> struct folio *folio) >>    * - folio_test_lru false >>    * - folio frozen (refcount of 0) >>    * >> - * Return: The lruvec this folio is on with its lock held. >> + * Return: The lruvec this folio is on with its lock held and rcu >> read lock held. >>    */ >>   struct lruvec *folio_lruvec_lock(struct folio *folio) >> +    __acquires(&lruvec->lru_lock) >> +    __acquires(rcu) >>   { >> -    struct lruvec *lruvec = folio_lruvec(folio); >> +    struct lruvec *lruvec; >> +    rcu_read_lock(); >> +retry: >> +    lruvec = folio_lruvec(folio); >>       spin_lock(&lruvec->lru_lock); >> -    lruvec_memcg_debug(lruvec, folio); >> +    if (unlikely(lruvec_memcg(lruvec) != folio_memcg(folio))) { >> +        spin_unlock(&lruvec->lru_lock); >> +        goto retry; >> +    } >>       return lruvec; >>   } >> @@ -1249,14 +1240,22 @@ struct lruvec *folio_lruvec_lock(struct folio >> *folio) >>    * - folio frozen (refcount of 0) >>    * >>    * Return: The lruvec this folio is on with its lock held and >> interrupts >> - * disabled. >> + * disabled and rcu read lock held. >>    */ >>   struct lruvec *folio_lruvec_lock_irq(struct folio *folio) >> +    __acquires(&lruvec->lru_lock) >> +    __acquires(rcu) >>   { >> -    struct lruvec *lruvec = folio_lruvec(folio); >> +    struct lruvec *lruvec; >> +    rcu_read_lock(); >> +retry: >> +    lruvec = folio_lruvec(folio); >>       spin_lock_irq(&lruvec->lru_lock); >> -    lruvec_memcg_debug(lruvec, folio); >> +    if (unlikely(lruvec_memcg(lruvec) != folio_memcg(folio))) { >> +        spin_unlock_irq(&lruvec->lru_lock); >> +        goto retry; >> +    } >>       return lruvec; >>   } >> @@ -1272,15 +1271,23 @@ struct lruvec *folio_lruvec_lock_irq(struct >> folio *folio) >>    * - folio frozen (refcount of 0) >>    * >>    * Return: The lruvec this folio is on with its lock held and >> interrupts >> - * disabled. >> + * disabled and rcu read lock held. >>    */ >>   struct lruvec *folio_lruvec_lock_irqsave(struct folio *folio, >>           unsigned long *flags) >> +    __acquires(&lruvec->lru_lock) >> +    __acquires(rcu) >>   { >> -    struct lruvec *lruvec = folio_lruvec(folio); >> +    struct lruvec *lruvec; >> +    rcu_read_lock(); >> +retry: >> +    lruvec = folio_lruvec(folio); >>       spin_lock_irqsave(&lruvec->lru_lock, *flags); >> -    lruvec_memcg_debug(lruvec, folio); >> +    if (unlikely(lruvec_memcg(lruvec) != folio_memcg(folio))) { >> +        spin_unlock_irqrestore(&lruvec->lru_lock, *flags); >> +        goto retry; >> +    } >>       return lruvec; >>   } >> diff --git a/mm/swap.c b/mm/swap.c >> index cb1148a92d8ec..7e53479ca1732 100644 >> --- a/mm/swap.c >> +++ b/mm/swap.c >> @@ -240,6 +240,7 @@ void folio_rotate_reclaimable(struct folio *folio) >>   void lru_note_cost_unlock_irq(struct lruvec *lruvec, bool file, >>           unsigned int nr_io, unsigned int nr_rotated) >>           __releases(lruvec->lru_lock) >> +        __releases(rcu) >>   { >>       unsigned long cost; >> @@ -253,6 +254,7 @@ void lru_note_cost_unlock_irq(struct lruvec >> *lruvec, bool file, >>       cost = nr_io * SWAP_CLUSTER_MAX + nr_rotated; >>       if (!cost) { >>           spin_unlock_irq(&lruvec->lru_lock); >> +        rcu_read_unlock(); > > Better to use lruvec_unlock_irq(lruvec)？ > >>           return; >>       } >> @@ -284,9 +286,11 @@ void lru_note_cost_unlock_irq(struct lruvec >> *lruvec, bool file, >>           } >>           spin_unlock_irq(&lruvec->lru_lock); >> +        rcu_read_unlock(); > > Ditto. > >>           lruvec = parent_lruvec(lruvec); >>           if (!lruvec) >>               break; >> +        rcu_read_lock(); >>           spin_lock_irq(&lruvec->lru_lock); > > lruvec_lock_irq(lruvec)? OK, will do. Thanks, Qi > > > Thanks. >>       } >>   } >