[PATCH] mm: fix a race on nr_swap_pages

[Zhaoyang Huang]

The scenario on which "Free swap -4kB" happens in my system, which is caused by
 get_swap_page_of_type or get_swap_pages racing with show_mem. Remove the race
 here.

Signed-off-by: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
---
 mm/swapfile.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index cf63b5f..13201b6 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -974,6 +974,8 @@ int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_size)
 	/* Only single cluster request supported */
 	WARN_ON_ONCE(n_goal > 1 && size == SWAPFILE_CLUSTER);
 
+	spin_lock(&swap_avail_lock);
+
 	avail_pgs = atomic_long_read(&nr_swap_pages) / size;
 	if (avail_pgs <= 0)
 		goto noswap;
@@ -986,8 +988,6 @@ int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_size)
 
 	atomic_long_sub(n_goal * size, &nr_swap_pages);
 
-	spin_lock(&swap_avail_lock);
-
 start_over:
 	node = numa_node_id();
 	plist_for_each_entry_safe(si, next, &swap_avail_heads[node], avail_lists[node]) {
@@ -1061,14 +1061,13 @@ swp_entry_t get_swap_page_of_type(int type)
 
 	spin_lock(&si->lock);
 	if (si->flags & SWP_WRITEOK) {
-		atomic_long_dec(&nr_swap_pages);
 		/* This is called for allocating swap entry, not cache */
 		offset = scan_swap_map(si, 1);
 		if (offset) {
+			atomic_long_dec(&nr_swap_pages);
 			spin_unlock(&si->lock);
 			return swp_entry(type, offset);
 		}
-		atomic_long_inc(&nr_swap_pages);
 	}
 	spin_unlock(&si->lock);
 fail:
-- 
1.9.1