Re: [PATCH RFC] mm: Add function for testing if the current lruvec lock is valid

[Alex Shi <alex.shi-KPsoFbNs7GizrGE5bRqYAgC/G2K4zDHf@public.gmane.org>]

It looks much better than mine. and could replace 'mm/lru: introduce the relock_page_lruvec function'
with your author signed. :)

BTW,
it's the rcu_read_lock cause the will-it-scale/page_fault3 regression which you mentained in another
letter?

Thanks
Alex

在 2020/8/1 上午5:14, alexander.h.duyck-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org 写道:
> From: Alexander Duyck <alexander.h.duyck-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>
> 
> When testing for relock we can avoid the need for RCU locking if we simply
> compare the page pgdat and memcg pointers versus those that the lruvec is
> holding. By doing this we can avoid the extra pointer walks and accesses of
> the memory cgroup.
> 
> In addition we can avoid the checks entirely if lruvec is currently NULL.
> 
> Signed-off-by: Alexander Duyck <alexander.h.duyck-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>
> ---
>  include/linux/memcontrol.h |   52 +++++++++++++++++++++++++++-----------------
>  1 file changed, 32 insertions(+), 20 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 6e670f991b42..7a02f00bf3de 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -405,6 +405,22 @@ static inline struct lruvec *mem_cgroup_lruvec(struct mem_cgroup *memcg,
>  
>  struct lruvec *mem_cgroup_page_lruvec(struct page *, struct pglist_data *);
>  
> +static inline bool lruvec_holds_page_lru_lock(struct page *page,
> +					      struct lruvec *lruvec)
> +{
> +	pg_data_t *pgdat = page_pgdat(page);
> +	const struct mem_cgroup *memcg;
> +	struct mem_cgroup_per_node *mz;
> +
> +	if (mem_cgroup_disabled())
> +		return lruvec == &pgdat->__lruvec;
> +
> +	mz = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
> +	memcg = page->mem_cgroup ? : root_mem_cgroup;
> +
> +	return lruvec->pgdat == pgdat && mz->memcg == memcg;
> +}
> +
>  struct mem_cgroup *mem_cgroup_from_task(struct task_struct *p);
>  
>  struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm);
> @@ -880,6 +896,14 @@ static inline struct lruvec *mem_cgroup_page_lruvec(struct page *page,
>  	return &pgdat->__lruvec;
>  }
>  
> +static inline bool lruvec_holds_page_lru_lock(struct page *page,
> +					      struct lruvec *lruvec)
> +{
> +		pg_data_t *pgdat = page_pgdat(page);
> +
> +		return lruvec == &pgdat->__lruvec;
> +}
> +
>  static inline struct mem_cgroup *parent_mem_cgroup(struct mem_cgroup *memcg)
>  {
>  	return NULL;
> @@ -1317,18 +1341,12 @@ static inline void unlock_page_lruvec_irqrestore(struct lruvec *lruvec,
>  static inline struct lruvec *relock_page_lruvec_irq(struct page *page,
>  		struct lruvec *locked_lruvec)
>  {
> -	struct pglist_data *pgdat = page_pgdat(page);
> -	bool locked;
> +	if (locked_lruvec) {
> +		if (lruvec_holds_page_lru_lock(page, locked_lruvec))
> +			return locked_lruvec;
>  
> -	rcu_read_lock();
> -	locked = mem_cgroup_page_lruvec(page, pgdat) == locked_lruvec;
> -	rcu_read_unlock();
> -
> -	if (locked)
> -		return locked_lruvec;
> -
> -	if (locked_lruvec)
>  		unlock_page_lruvec_irq(locked_lruvec);
> +	}
>  
>  	return lock_page_lruvec_irq(page);
>  }
> @@ -1337,18 +1355,12 @@ static inline struct lruvec *relock_page_lruvec_irq(struct page *page,
>  static inline struct lruvec *relock_page_lruvec_irqsave(struct page *page,
>  		struct lruvec *locked_lruvec, unsigned long *flags)
>  {
> -	struct pglist_data *pgdat = page_pgdat(page);
> -	bool locked;
> -
> -	rcu_read_lock();
> -	locked = mem_cgroup_page_lruvec(page, pgdat) == locked_lruvec;
> -	rcu_read_unlock();
> -
> -	if (locked)
> -		return locked_lruvec;
> +	if (locked_lruvec) {
> +		if (lruvec_holds_page_lru_lock(page, locked_lruvec))
> +			return locked_lruvec;
>  
> -	if (locked_lruvec)
>  		unlock_page_lruvec_irqrestore(locked_lruvec, *flags);
> +	}
>  
>  	return lock_page_lruvec_irqsave(page, flags);
>  }
>