Messages in this thread | | | Date | Wed, 25 May 2022 10:59:58 -0400 | Subject | Re: [PATCH v4 03/11] mm: memcontrol: make lruvec lock safe when LRU pages are reparented | From | Waiman Long <> |
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On 5/25/22 06:20, Muchun Song wrote: > On Tue, May 24, 2022 at 03:23:11PM -0400, Waiman Long wrote: >> On 5/24/22 02:05, Muchun Song wrote: >>> The diagram below shows how to make the folio lruvec lock safe when LRU >>> pages are reparented. >>> >>> folio_lruvec_lock(folio) >>> retry: >>> lruvec = folio_lruvec(folio); >>> >>> // The folio is reparented at this time. >>> spin_lock(&lruvec->lru_lock); >>> >>> if (unlikely(lruvec_memcg(lruvec) != folio_memcg(folio))) >>> // Acquired the wrong lruvec lock and need to retry. >>> // Because this folio is on the parent memcg lruvec list. >>> goto retry; >>> >>> // If we reach here, it means that folio_memcg(folio) is stable. >>> >>> memcg_reparent_objcgs(memcg) >>> // lruvec belongs to memcg and lruvec_parent belongs to parent memcg. >>> spin_lock(&lruvec->lru_lock); >>> spin_lock(&lruvec_parent->lru_lock); >>> >>> // Move all the pages from the lruvec list to the parent lruvec list. >>> >>> spin_unlock(&lruvec_parent->lru_lock); >>> spin_unlock(&lruvec->lru_lock); >>> >>> After we acquire the lruvec lock, we need to check whether the folio is >>> reparented. If so, we need to reacquire the new lruvec lock. On the >>> routine of the LRU pages reparenting, we will also acquire the lruvec >>> lock (will be implemented in the later patch). So folio_memcg() cannot >>> be changed when we hold the lruvec lock. >>> >>> Since lruvec_memcg(lruvec) is always equal to folio_memcg(folio) after >>> we hold the lruvec lock, lruvec_memcg_debug() check is pointless. So >>> remove it. >>> >>> This is a preparation for reparenting the LRU pages. >>> >>> Signed-off-by: Muchun Song <songmuchun@bytedance.com> >>> --- >>> include/linux/memcontrol.h | 18 +++----------- >>> mm/compaction.c | 10 +++++++- >>> mm/memcontrol.c | 62 +++++++++++++++++++++++++++++----------------- >>> mm/swap.c | 4 +++ >>> 4 files changed, 55 insertions(+), 39 deletions(-) >>> >>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h >>> index ff1c1dd7e762..4042e4d21fe2 100644 >>> --- a/include/linux/memcontrol.h >>> +++ b/include/linux/memcontrol.h >>> @@ -752,7 +752,9 @@ 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. >>> + * The lruvec can be changed to its parent lruvec when the page reparented. >>> + * The caller need to recheck if it cares about this changes (just like >>> + * folio_lruvec_lock() does). >>> */ >>> static inline struct lruvec *folio_lruvec(struct folio *folio) >>> { >>> @@ -771,15 +773,6 @@ 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; >>> @@ -1240,11 +1233,6 @@ 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; >>> diff --git a/mm/compaction.c b/mm/compaction.c >>> index 817098817302..1692b17db781 100644 >>> --- a/mm/compaction.c >>> +++ b/mm/compaction.c >>> @@ -515,6 +515,8 @@ compact_folio_lruvec_lock_irqsave(struct folio *folio, unsigned long *flags, >>> { >>> struct lruvec *lruvec; >>> + rcu_read_lock(); >>> +retry: >>> lruvec = folio_lruvec(folio); >>> /* Track if the lock is contended in async mode */ >>> @@ -527,7 +529,13 @@ compact_folio_lruvec_lock_irqsave(struct folio *folio, unsigned long *flags, >>> spin_lock_irqsave(&lruvec->lru_lock, *flags); >>> out: >>> - lruvec_memcg_debug(lruvec, folio); >>> + if (unlikely(lruvec_memcg(lruvec) != folio_memcg(folio))) { >>> + spin_unlock_irqrestore(&lruvec->lru_lock, *flags); >>> + goto retry; >>> + } >>> + >>> + /* See the comments in folio_lruvec_lock(). */ >>> + rcu_read_unlock(); >>> return lruvec; >>> } >>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c >>> index 6de0d3e53eb1..b38a77f6696f 100644 >>> --- a/mm/memcontrol.c >>> +++ b/mm/memcontrol.c >>> @@ -1199,23 +1199,6 @@ int mem_cgroup_scan_tasks(struct mem_cgroup *memcg, >>> return ret; >>> } >>> -#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(lruvec_memcg(lruvec) != root_mem_cgroup, 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. >>> @@ -1230,10 +1213,23 @@ void lruvec_memcg_debug(struct lruvec *lruvec, struct folio *folio) >>> */ >>> struct lruvec *folio_lruvec_lock(struct folio *folio) >>> { >>> - 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; >>> + } >>> + >>> + /* >>> + * Preemption is disabled in the internal of spin_lock, which can serve >>> + * as RCU read-side critical sections. >>> + */ >> What is the point of this comment as preemption is not disabled for >> PREEMPT_RT kernel? >> > I'm not familar with PREEMPT_RT kernel. At least you are right, > preemption is not disabled in this case, I think I should drop > this assumption.
Preemption is not disabled for PREEMPT_RT kernel but task migration to another cpu is disabled. So access to per-cpu variables are safe. RCU seems to have a special mode for PREEMPT_RT kernel but I am not familiar with the detail.
Cheers, Longman
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