mm/swap_state.c: simplify total_swapcache_pages() with get_swap_device()

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.

[ying.huang@intel.com: fix bad swap file entry warning]
  Link: http://lkml.kernel.org/r/20190531024102.21723-1-ying.huang@intel.com
Link: http://lkml.kernel.org/r/20190527082714.12151-1-ying.huang@intel.comSigned-off-by: "Huang, Ying" <ying.huang@intel.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: Mike Kravetz <mike.kravetz@oracle.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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/swap_state.c

@@ -73,23 +73,24 @@ 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)
+		swp_entry_t entry = swp_entry(i, 1);
+
+		/* Avoid get_swap_device() to warn for bad swap entry */
+		if (!swp_swap_info(entry))
+			continue;
+		/* Prevent swapoff to free swapper_spaces */
+		si = get_swap_device(entry);
+		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 +612,16 @@ 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,