[PATCH] swap: Check mapping_empty() for swap cache before being freed

[Huang Ying]

To check whether all pages and shadow entries in swap cache has been
removed before swap cache is freed.

Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
Cc: Miaohe Lin <linmiaohe@xxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Ilya Dryomov <idryomov@xxxxxxxxx>
---

BTW: The patch reverts the following patch in -mm tree in effect.

mm/swap: remove unused global variable nr_swapper_spaces

---
 mm/swap_state.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/mm/swap_state.c b/mm/swap_state.c
index 95e391f46468..c56aa9ac050d 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -37,6 +37,7 @@ static const struct address_space_operations swap_aops = {
 };
 
 struct address_space *swapper_spaces[MAX_SWAPFILES] __read_mostly;
+static unsigned int nr_swapper_spaces[MAX_SWAPFILES] __read_mostly;
 static bool enable_vma_readahead __read_mostly = true;
 
 #define SWAP_RA_WIN_SHIFT	(PAGE_SHIFT / 2)
@@ -684,6 +685,7 @@ int init_swap_address_space(unsigned int type, unsigned long nr_pages)
 		/* swap cache doesn't use writeback related tags */
 		mapping_set_no_writeback_tags(space);
 	}
+	nr_swapper_spaces[type] = nr;
 	swapper_spaces[type] = spaces;
 
 	return 0;
@@ -691,7 +693,13 @@ int init_swap_address_space(unsigned int type, unsigned long nr_pages)
 
 void exit_swap_address_space(unsigned int type)
 {
-	kvfree(swapper_spaces[type]);
+	int i;
+	struct address_space *spaces = swapper_spaces[type];
+
+	for (i = 0; i < nr_swapper_spaces[type]; i++)
+		VM_WARN_ON_ONCE(!mapping_empty(&spaces[i]));
+	kvfree(spaces);
+	nr_swapper_spaces[type] = 0;
 	swapper_spaces[type] = NULL;
 }
 
-- 
2.30.2