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Date:   Fri, 3 Jan 2020 11:29:06 -0800 (PST)
From:   David Rientjes <rientjes@google.com>
To:     Wei Yang <richardw.yang@linux.intel.com>
cc:     hannes@cmpxchg.org, mhocko@kernel.org, vdavydov.dev@gmail.com,
        akpm@linux-foundation.org, kirill.shutemov@linux.intel.com,
        cgroups@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, yang.shi@linux.alibaba.com
Subject: Re: [RFC PATCH] mm: thp: grab the lock before manipulation defer
 list
In-Reply-To: <20200103143407.1089-1-richardw.yang@linux.intel.com>
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References: <20200103143407.1089-1-richardw.yang@linux.intel.com>
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On Fri, 3 Jan 2020, Wei Yang wrote:

> As all the other places, we grab the lock before manipulate the defer list.
> Current implementation may face a race condition.
> 
> Fixes: 87eaceb3faa5 ("mm: thp: make deferred split shrinker memcg aware")
> 
> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
> 
> ---
> I notice the difference during code reading and just confused about the
> difference. No specific test is done since limited knowledge about cgroup.
> 
> Maybe I miss something important?

The check for !list_empty(page_deferred_list(page)) must certainly be 
serialized with doing list_del_init(page_deferred_list(page)).

> ---
>  mm/memcontrol.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index bc01423277c5..62b7ec34ef1a 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5368,12 +5368,12 @@ static int mem_cgroup_move_account(struct page *page,
>  	}
>  
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +	spin_lock(&from->deferred_split_queue.split_queue_lock);
>  	if (compound && !list_empty(page_deferred_list(page))) {
> -		spin_lock(&from->deferred_split_queue.split_queue_lock);
>  		list_del_init(page_deferred_list(page));
>  		from->deferred_split_queue.split_queue_len--;
> -		spin_unlock(&from->deferred_split_queue.split_queue_lock);
>  	}
> +	spin_unlock(&from->deferred_split_queue.split_queue_lock);
>  #endif
>  	/*
>  	 * It is safe to change page->mem_cgroup here because the page
> @@ -5385,13 +5385,13 @@ static int mem_cgroup_move_account(struct page *page,
>  	page->mem_cgroup = to;
>  
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +	spin_lock(&to->deferred_split_queue.split_queue_lock);
>  	if (compound && list_empty(page_deferred_list(page))) {
> -		spin_lock(&to->deferred_split_queue.split_queue_lock);
>  		list_add_tail(page_deferred_list(page),
>  			      &to->deferred_split_queue.split_queue);
>  		to->deferred_split_queue.split_queue_len++;
> -		spin_unlock(&to->deferred_split_queue.split_queue_lock);
>  	}
> +	spin_unlock(&to->deferred_split_queue.split_queue_lock);
>  #endif
>  
>  	spin_unlock_irqrestore(&from->move_lock, flags);
> -- 
> 2.17.1
> 
> 
>