Messages in this thread | | | Date | Tue, 24 May 2022 15:23:11 -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 5/24/22 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?
> + rcu_read_unlock(); > > return lruvec; > } > @@ -1253,10 +1249,20 @@ struct lruvec *folio_lruvec_lock(struct folio *folio) > */ > struct lruvec *folio_lruvec_lock_irq(struct folio *folio) > { > - 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; > + } > + > + /* See the comments in folio_lruvec_lock(). */ > + rcu_read_unlock(); > > return lruvec; > } > @@ -1278,10 +1284,20 @@ struct lruvec *folio_lruvec_lock_irq(struct folio *folio) > struct lruvec *folio_lruvec_lock_irqsave(struct folio *folio, > unsigned long *flags) > { > - 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; > + } > + > + /* See the comments in folio_lruvec_lock(). */ > + rcu_read_unlock(); > > return lruvec; > } > diff --git a/mm/swap.c b/mm/swap.c > index 7e320ec08c6a..9680f2fc48b1 100644 > --- a/mm/swap.c > +++ b/mm/swap.c > @@ -303,6 +303,10 @@ void lru_note_cost(struct lruvec *lruvec, bool file, unsigned int nr_pages) > > void lru_note_cost_folio(struct folio *folio) > { > + /* > + * The rcu read lock is held by the caller, so we do not need to > + * care about the lruvec returned by folio_lruvec() being released. > + */ Maybe we can add "WARN_ON_ONCE(!rcu_read_lock_held())" to be sure.
> lru_note_cost(folio_lruvec(folio), folio_is_file_lru(folio), > folio_nr_pages(folio)); > }
Cheers, Longman
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