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

[Johannes Weiner]

On Tue, May 24, 2022 at 02:05:43PM +0800, 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@xxxxxxxxxxxxx>

This looks good to me. Just one question:

> @@ -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.
> +	 */
> +	rcu_read_unlock();

The code looks right to me, but I don't understand the comment: why do
we care that the rcu read-side continues? With the lru_lock held,
reparenting is on hold and the lruvec cannot be rcu-freed anyway, no?