Re: [PATCH -mm] mm, swap: Simplify total_swapcache_pages() with get_swap_device()

["Paul E. McKenney" <paulmck@linux.ibm.com>]

On Mon, May 27, 2019 at 04:27:14PM +0800, Huang, Ying wrote:
> From: Huang Ying <ying.huang@intel.com>
> 
> total_swapcache_pages() may race with swapper_spaces[] allocation and
> freeing.  Previously, this is protected with a swapper_spaces[]
> specific RCU mechanism.  To simplify the logic/code complexity, it is
> replaced with get/put_swap_device().  The code line number is reduced
> too.  Although not so important, the swapoff() performance improves
> too because one synchronize_rcu() call during swapoff() is deleted.

I am guessing that total_swapcache_pages() is not used on any
fastpaths, but must defer to others on this.  Of course, if the
performance/scalability of total_swapcache_pages() is important,
benchmarking is needed.

But where do I find get_swap_device() and put_swap_device()?  I do not
see them in current mainline.

							Thanx, Paul

> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Tim Chen <tim.c.chen@linux.intel.com>
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Cc: Jérôme Glisse <jglisse@redhat.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Yang Shi <yang.shi@linux.alibaba.com>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Dave Jiang <dave.jiang@intel.com>
> Cc: Daniel Jordan <daniel.m.jordan@oracle.com>
> Cc: Andrea Parri <andrea.parri@amarulasolutions.com>
> ---
>  mm/swap_state.c | 28 ++++++++++------------------
>  1 file changed, 10 insertions(+), 18 deletions(-)
> 
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index f509cdaa81b1..b84c58b572ca 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -73,23 +73,19 @@ unsigned long total_swapcache_pages(void)
>  	unsigned int i, j, nr;
>  	unsigned long ret = 0;
>  	struct address_space *spaces;
> +	struct swap_info_struct *si;
> 
> -	rcu_read_lock();
>  	for (i = 0; i < MAX_SWAPFILES; i++) {
> -		/*
> -		 * The corresponding entries in nr_swapper_spaces and
> -		 * swapper_spaces will be reused only after at least
> -		 * one grace period.  So it is impossible for them
> -		 * belongs to different usage.
> -		 */
> -		nr = nr_swapper_spaces[i];
> -		spaces = rcu_dereference(swapper_spaces[i]);
> -		if (!nr || !spaces)
> +		/* Prevent swapoff to free swapper_spaces */
> +		si = get_swap_device(swp_entry(i, 1));
> +		if (!si)
>  			continue;
> +		nr = nr_swapper_spaces[i];
> +		spaces = swapper_spaces[i];
>  		for (j = 0; j < nr; j++)
>  			ret += spaces[j].nrpages;
> +		put_swap_device(si);
>  	}
> -	rcu_read_unlock();
>  	return ret;
>  }
> 
> @@ -611,20 +607,16 @@ int init_swap_address_space(unsigned int type, unsigned long nr_pages)
>  		mapping_set_no_writeback_tags(space);
>  	}
>  	nr_swapper_spaces[type] = nr;
> -	rcu_assign_pointer(swapper_spaces[type], spaces);
> +	swapper_spaces[type] = spaces;
> 
>  	return 0;
>  }
> 
>  void exit_swap_address_space(unsigned int type)
>  {
> -	struct address_space *spaces;
> -
> -	spaces = swapper_spaces[type];
> +	kvfree(swapper_spaces[type]);
>  	nr_swapper_spaces[type] = 0;
> -	rcu_assign_pointer(swapper_spaces[type], NULL);
> -	synchronize_rcu();
> -	kvfree(spaces);
> +	swapper_spaces[type] = NULL;
>  }
> 
>  static inline void swap_ra_clamp_pfn(struct vm_area_struct *vma,
> -- 
> 2.20.1
>