linux-kernel - Re: [PATCH 2/2] mm/swap: Remove lock_initialized flag from swap_slots

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Date:   Mon, 24 Jul 2017 10:15:29 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Ying Huang <ying.huang@...el.com>,
        Wenwei Tao <wenwei.tww@...baba-inc.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, Minchan Kim <minchan@...nel.org>,
        "Rik van Riel" <riel@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Johannes Weiner" <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Hillf Danton <hillf.zj@...baba-inc.com>
Subject: Re: [PATCH 2/2] mm/swap: Remove lock_initialized flag from swap_slots_cache

Hi, Tim,

Tim Chen <tim.c.chen@...ux.intel.com> writes:

> We will only reach the lock initialization code
> in alloc_swap_slot_cache when the cpu's swap_slots_cache's slots
> have not been allocated and swap_slots_cache has not been initialized
> previously.  So the lock_initialized check is redundant and unnecessary.
> Remove lock_initialized flag from swap_slots_cache to save memory.

Is there a race condition with CPU offline/online when preempt is enabled?

CPU A                                   CPU B
-----                                   -----
                                        get_swap_page()
                                          get cache[B], cache[B]->slots != NULL
                                          preempted and moved to CPU A
                                        be offlined
                                        be onlined
                                          alloc_swap_slot_cache()
mutex_lock(cache[B]->alloc_lock)
                                            mutex_init(cache[B]->alloc_lock) !!!

The cache[B]->alloc_lock will be reinitialized when it is still held.

Best Regards,
Huang, Ying

> Reported-by: Wenwei Tao <wenwei.tww@...baba-inc.com>
> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
> ---
>  include/linux/swap_slots.h | 1 -
>  mm/swap_slots.c            | 9 ++++-----
>  2 files changed, 4 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/swap_slots.h b/include/linux/swap_slots.h
> index 6ef92d1..a75c30b 100644
> --- a/include/linux/swap_slots.h
> +++ b/include/linux/swap_slots.h
> @@ -10,7 +10,6 @@
>  #define THRESHOLD_DEACTIVATE_SWAP_SLOTS_CACHE	(2*SWAP_SLOTS_CACHE_SIZE)
>  
>  struct swap_slots_cache {
> -	bool		lock_initialized;
>  	struct mutex	alloc_lock; /* protects slots, nr, cur */
>  	swp_entry_t	*slots;
>  	int		nr;
> diff --git a/mm/swap_slots.c b/mm/swap_slots.c
> index 4c5457c..c039e6c 100644
> --- a/mm/swap_slots.c
> +++ b/mm/swap_slots.c
> @@ -140,11 +140,10 @@ static int alloc_swap_slot_cache(unsigned int cpu)
>  	if (cache->slots || cache->slots_ret)
>  		/* cache already allocated */
>  		goto out;
> -	if (!cache->lock_initialized) {
> -		mutex_init(&cache->alloc_lock);
> -		spin_lock_init(&cache->free_lock);
> -		cache->lock_initialized = true;
> -	}
> +
> +	mutex_init(&cache->alloc_lock);
> +	spin_lock_init(&cache->free_lock);
> +
>  	cache->nr = 0;
>  	cache->cur = 0;
>  	cache->n_ret = 0;