Re: [PATCH 2/3] mm/memcg: remove rcu locking for lock_page_lruvec function series

[Hugh Dickins]

On Thu, 17 Dec 2020, Alex Shi wrote:

> The rcu_read_lock was used to block memcg destory, but with the detailed
> calling conditions, the memcg won't gone since the page is hold. So we
> don't need it now, let's remove them to save locking load in debugging.

"
lock_page_lruvec() and its variants used rcu_read_lock() with the
intention of safeguarding against the mem_cgroup being destroyed
concurrently; but so long as they are called under the specified
conditions (as they are), there is no way for the page's mem_cgroup
to be destroyed.  Delete the unnecessary rcu_read_lock() and _unlock().
"

This has little to do with a "locking load in debugging" - so what?
But everything to do with deleting bogosity, the sooner the better.

> 
> Signed-off-by: Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>

Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>

This really surprised me!  Nice change, but how on earth did we not
notice until now?  The rcu_read_lock() seems to have come in, without
explanation, somewhere between lru_lock v9 and v11 (I never saw v10); and
I guess I was so used to needing rcu_read_lock() in my own implementation,
that I was blind to its irrelevance in yours.  Cc'ing Alex Duyck, since
he was generally very alert to this kind of thing - be good to have his
Ack too.  Also Cc'ing Hui Su, who sent a similar but unexplained patch
just before yours.

> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: cgroups@xxxxxxxxxxxxxxx
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
>  mm/memcontrol.c | 6 ------
>  1 file changed, 6 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index e6b50d068b2f..98bbee1d2faf 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1356,10 +1356,8 @@ struct lruvec *lock_page_lruvec(struct page *page)
>  	struct lruvec *lruvec;
>  	struct pglist_data *pgdat = page_pgdat(page);
>  
> -	rcu_read_lock();
>  	lruvec = mem_cgroup_page_lruvec(page, pgdat);
>  	spin_lock(&lruvec->lru_lock);
> -	rcu_read_unlock();
>  
>  	lruvec_memcg_debug(lruvec, page);
>  
> @@ -1371,10 +1369,8 @@ struct lruvec *lock_page_lruvec_irq(struct page *page)
>  	struct lruvec *lruvec;
>  	struct pglist_data *pgdat = page_pgdat(page);
>  
> -	rcu_read_lock();
>  	lruvec = mem_cgroup_page_lruvec(page, pgdat);
>  	spin_lock_irq(&lruvec->lru_lock);
> -	rcu_read_unlock();
>  
>  	lruvec_memcg_debug(lruvec, page);
>  
> @@ -1386,10 +1382,8 @@ struct lruvec *lock_page_lruvec_irqsave(struct page *page, unsigned long *flags)
>  	struct lruvec *lruvec;
>  	struct pglist_data *pgdat = page_pgdat(page);
>  
> -	rcu_read_lock();
>  	lruvec = mem_cgroup_page_lruvec(page, pgdat);
>  	spin_lock_irqsave(&lruvec->lru_lock, *flags);
> -	rcu_read_unlock();
>  
>  	lruvec_memcg_debug(lruvec, page);
>  
> -- 
> 2.29.GIT