Re: [PATCH v4 03/11] mm: memcontrol: make lruvec lock safe when LRU pages are reparented

[Waiman Long <longman@redhat.com>]

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