linux-kernel - Re: [PATCH v2 2/3] mm/memcg: remove rcu locking for lock_page

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Message-ID: <398f089d-250b-39f7-691a-9ce495b30700@linux.alibaba.com>
Date:   Tue, 22 Dec 2020 16:00:20 +0800
From:   Alex Shi <alex.shi@...ux.alibaba.com>
To:     hughd@...gle.com
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Hui Su <sh_def@....com>,
        Alexander Duyck <alexander.duyck@...il.com>
Subject: Re: [PATCH v2 2/3] mm/memcg: remove rcu locking for lock_page_lruvec
 function series

Cc: Hui Su and Alexander Duyck as Hugh suggested.

在 2020/12/22 下午1:20, Alex Shi 写道:
> 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().
> 
> Hugh Dickin's polished the commit log, Thanks a lot!
> 
> Signed-off-by: Alex Shi <alex.shi@...ux.alibaba.com>
> Acked-by: Hugh Dickins <hughd@...gle.com>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Michal Hocko <mhocko@...nel.org>
> Cc: Vladimir Davydov <vdavydov.dev@...il.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: cgroups@...r.kernel.org
> Cc: linux-mm@...ck.org
> Cc: linux-kernel@...r.kernel.org
> ---
>  mm/memcontrol.c | 6 ------
>  1 file changed, 6 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 8d400efc81b9..0af13c4fe4b3 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);
>  
>