Re: [PATCH v3 3/7] mm/lru: replace pgdat lru_lock with lruvec lock

[Johannes Weiner <hannes@cmpxchg.org>]

On Sat, Nov 16, 2019 at 11:15:02AM +0800, Alex Shi wrote:
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 62470325f9bc..cf274739e619 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1246,6 +1246,42 @@ struct lruvec *mem_cgroup_page_lruvec(struct page *page, struct pglist_data *pgd
>  	return lruvec;
>  }
>  
> +struct lruvec *lock_page_lruvec_irq(struct page *page,
> +					struct pglist_data *pgdat)
> +{
> +	struct lruvec *lruvec;
> +
> +again:
> +	lruvec = mem_cgroup_page_lruvec(page, pgdat);
> +	spin_lock_irq(&lruvec->lru_lock);

This isn't safe. Nothing prevents the page from being moved to a
different memcg in between these two operations, and the lruvec having
been freed by the time you try to acquire the spinlock.

You need to use the rcu lock to dereference page->mem_cgroup and its
lruvec when coming from the page like this.

You also need to use page_memcg_rcu() to insert the appropriate
lockless access barriers, which mem_cgroup_page_lruvec() does not do
since it's designed for use with pgdat->lru_lock.