Re: [PATCH v3 20/21] mm: split zone->lru_lock

[Konstantin Khlebnikov <khlebnikov@openvz.org>]

KAMEZAWA Hiroyuki wrote:
> On Thu, 23 Feb 2012 17:53:23 +0400
> Konstantin Khlebnikov<khlebnikov@openvz.org>  wrote:
>
>> Looks like all ready for splitting zone->lru_lock into per-lruvec pieces.
>>
>> lruvec locking loop protected with rcu, actually there is irq-disabling instead
>> of rcu_read_lock(). Memory controller already releases its lru-vectors after
>> syncronize_rcu() in cgroup_diput(). Probably it should be replaced with synchronize_sched()
>>
>> Signed-off-by: Konstantin Khlebnikov<khlebnikov@openvz.org>
>
> Do we need rcu_read_lock() even if we check isolated pages at pre_destroy() ?
> If pre_destroy() ends, pages under a memcg being destroyed were moved to other
> cgroup while it's usolated.
>
> So,
>   - PageLRU(page) guarantees lruvec is valid.
>   - if !PageLRU(page), the caller of lru_lock should know what it does.
>     Once isolated, pre_destroy() never ends and page_lruvec(page) is always stable.

This will be racy, because lruvec can be changed and released between checking PageLRU() and page_lruvec() or
between page_lruvec() and spin_lock(), so we need to protect whole operation with single locking interval.

>
> Thanks,
> -Kame
>
>> ---
>>   include/linux/mmzone.h |    3 +-
>>   mm/compaction.c        |    2 +
>>   mm/internal.h          |   66 +++++++++++++++++++++++++-----------------------
>>   mm/page_alloc.c        |    2 +
>>   mm/swap.c              |    2 +
>>   5 files changed, 40 insertions(+), 35 deletions(-)
>>
>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>> index 2e3a298..9880150 100644
>> --- a/include/linux/mmzone.h
>> +++ b/include/linux/mmzone.h
>> @@ -304,6 +304,8 @@ struct zone_reclaim_stat {
>>   };
>>
>>   struct lruvec {
>> +	spinlock_t		lru_lock;
>> +
>>   	struct list_head	pages_lru[NR_LRU_LISTS];
>>   	unsigned long		pages_count[NR_LRU_COUNTERS];
>>
>> @@ -386,7 +388,6 @@ struct zone {
>>   	ZONE_PADDING(_pad1_)
>>
>>   	/* Fields commonly accessed by the page reclaim scanner */
>> -	spinlock_t		lru_lock;
>>   	struct lruvec		lruvec;
>>
>>   	unsigned long		pages_scanned;	   /* since last reclaim */
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index fa74cbe..8661bb58 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -306,7 +306,7 @@ static isolate_migrate_t isolate_migratepages(struct zone *zone,
>>   			lruvec = NULL;
>>   		}
>>   		if (need_resched() ||
>> -		    (lruvec&&  spin_is_contended(&zone->lru_lock))) {
>> +		    (lruvec&&  spin_is_contended(&lruvec->lru_lock))) {
>>   			if (lruvec)
>>   				unlock_lruvec_irq(lruvec);
>>   			lruvec = NULL;
>> diff --git a/mm/internal.h b/mm/internal.h
>> index 6dd2e70..9a9fd53 100644
>> --- a/mm/internal.h
>> +++ b/mm/internal.h
>> @@ -15,27 +15,27 @@
>>
>>   static inline void lock_lruvec(struct lruvec *lruvec, unsigned long *flags)
>>   {
>> -	spin_lock_irqsave(&lruvec_zone(lruvec)->lru_lock, *flags);
>> +	spin_lock_irqsave(&lruvec->lru_lock, *flags);
>>   }
>>
>>   static inline void lock_lruvec_irq(struct lruvec *lruvec)
>>   {
>> -	spin_lock_irq(&lruvec_zone(lruvec)->lru_lock);
>> +	spin_lock_irq(&lruvec->lru_lock);
>>   }
>>
>>   static inline void unlock_lruvec(struct lruvec *lruvec, unsigned long *flags)
>>   {
>> -	spin_unlock_irqrestore(&lruvec_zone(lruvec)->lru_lock, *flags);
>> +	spin_unlock_irqrestore(&lruvec->lru_lock, *flags);
>>   }
>>
>>   static inline void unlock_lruvec_irq(struct lruvec *lruvec)
>>   {
>> -	spin_unlock_irq(&lruvec_zone(lruvec)->lru_lock);
>> +	spin_unlock_irq(&lruvec->lru_lock);
>>   }
>>
>>   static inline void wait_lruvec_unlock(struct lruvec *lruvec)
>>   {
>> -	spin_unlock_wait(&lruvec_zone(lruvec)->lru_lock);
>> +	spin_unlock_wait(&lruvec->lru_lock);
>>   }
>>
>>   #ifdef CONFIG_CGROUP_MEM_RES_CTLR
>> @@ -46,37 +46,39 @@ static inline void wait_lruvec_unlock(struct lruvec *lruvec)
>>   static inline struct lruvec *__relock_page_lruvec(struct lruvec *locked_lruvec,
>>   						  struct page *page)
>>   {
>> -	/* Currenyly only one lru_lock per-zone */
>> -	return page_lruvec(page);
>> +	struct lruvec *lruvec;
>> +
>> +	do {
>> +		lruvec = page_lruvec(page);
>> +		if (likely(lruvec == locked_lruvec))
>> +			return lruvec;
>> +		spin_unlock(&locked_lruvec->lru_lock);
>> +		spin_lock(&lruvec->lru_lock);
>> +		locked_lruvec = lruvec;
>> +	} while (1);
>>   }
>>
>>   static inline struct lruvec *relock_page_lruvec_irq(struct lruvec *lruvec,
>>   						    struct page *page)
>>   {
>> -	struct zone *zone = page_zone(page);
>> -
>>   	if (!lruvec) {
>> -		spin_lock_irq(&zone->lru_lock);
>> -	} else if (zone != lruvec_zone(lruvec)) {
>> -		unlock_lruvec_irq(lruvec);
>> -		spin_lock_irq(&zone->lru_lock);
>> +		local_irq_disable();
>> +		lruvec = page_lruvec(page);
>> +		spin_lock(&lruvec->lru_lock);
>>   	}
>> -	return page_lruvec(page);
>> +	return __relock_page_lruvec(lruvec, page);
>>   }
>>
>>   static inline struct lruvec *relock_page_lruvec(struct lruvec *lruvec,
>>   						struct page *page,
>>   						unsigned long *flags)
>>   {
>> -	struct zone *zone = page_zone(page);
>> -
>>   	if (!lruvec) {
>> -		spin_lock_irqsave(&zone->lru_lock, *flags);
>> -	} else if (zone != lruvec_zone(lruvec)) {
>> -		unlock_lruvec(lruvec, flags);
>> -		spin_lock_irqsave(&zone->lru_lock, *flags);
>> +		local_irq_save(*flags);
>> +		lruvec = page_lruvec(page);
>> +		spin_lock(&lruvec->lru_lock);
>>   	}
>> -	return page_lruvec(page);
>> +	return __relock_page_lruvec(lruvec, page);
>>   }
>>
>>   /*
>> @@ -87,22 +89,24 @@ static inline struct lruvec *relock_page_lruvec(struct lruvec *lruvec,
>>   static inline bool __lock_page_lruvec_irq(struct lruvec **lruvec,
>>   					  struct page *page)
>>   {
>> -	struct zone *zone;
>>   	bool ret = false;
>>
>> +	rcu_read_lock();
>> +	/*
>> +	 * If we see there PageLRU(), it means page has valid lruvec link.
>> +	 * We need protect whole operation with single rcu-interval, otherwise
>> +	 * lruvec which hold this LRU sign can run out before we secure it.
>> +	 */
>>   	if (PageLRU(page)) {
>>   		if (!*lruvec) {
>> -			zone = page_zone(page);
>> -			spin_lock_irq(&zone->lru_lock);
>> -		} else
>> -			zone = lruvec_zone(*lruvec);
>> -
>> -		if (PageLRU(page)) {
>>   			*lruvec = page_lruvec(page);
>> +			lock_lruvec_irq(*lruvec);
>> +		}
>> +		*lruvec = __relock_page_lruvec(*lruvec, page);
>> +		if (PageLRU(page))
>>   			ret = true;
>> -		} else
>> -			*lruvec =&zone->lruvec;
>>   	}
>> +	rcu_read_unlock();
>>
>>   	return ret;
>>   }
>> @@ -110,7 +114,7 @@ static inline bool __lock_page_lruvec_irq(struct lruvec **lruvec,
>>   /* Wait for lruvec unlock before locking other lruvec for the same page */
>>   static inline void __wait_lruvec_unlock(struct lruvec *lruvec)
>>   {
>> -	/* Currently only one lru_lock per-zone */
>> +	wait_lruvec_unlock(lruvec);
>>   }
>>
>>   #else /* CONFIG_CGROUP_MEM_RES_CTLR */
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index ab42446..beadcc9 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -4294,6 +4294,7 @@ void init_zone_lruvec(struct zone *zone, struct lruvec *lruvec)
>>   	enum lru_list lru;
>>
>>   	memset(lruvec, 0, sizeof(struct lruvec));
>> +	spin_lock_init(&lruvec->lru_lock);
>>   	for_each_lru(lru)
>>   		INIT_LIST_HEAD(&lruvec->pages_lru[lru]);
>>   #ifdef CONFIG_CGROUP_MEM_RES_CTLR
>> @@ -4369,7 +4370,6 @@ static void __paginginit free_area_init_core(struct pglist_data *pgdat,
>>   #endif
>>   		zone->name = zone_names[j];
>>   		spin_lock_init(&zone->lock);
>> -		spin_lock_init(&zone->lru_lock);
>>   		zone_seqlock_init(zone);
>>   		zone->zone_pgdat = pgdat;
>>
>> diff --git a/mm/swap.c b/mm/swap.c
>> index 998c71c..8156181 100644
>> --- a/mm/swap.c
>> +++ b/mm/swap.c
>> @@ -700,7 +700,7 @@ void lru_add_page_tail(struct lruvec *lruvec,
>>   	VM_BUG_ON(!PageHead(page));
>>   	VM_BUG_ON(PageCompound(page_tail));
>>   	VM_BUG_ON(PageLRU(page_tail));
>> -	VM_BUG_ON(NR_CPUS != 1&&  !spin_is_locked(&lruvec_zone(lruvec)->lru_lock));
>> +	VM_BUG_ON(NR_CPUS != 1&&  !spin_is_locked(&lruvec->lru_lock));
>>
>>   	SetPageLRU(page_tail);
>>
>>
>>
>

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