[PATCH] mm: do not free shared swap slots

[Vinayak Menon <vinmenon@codeaurora.org>]

The following race is observed due to which a processes faulting
on a swap entry, finds the page neither in swapcache nor swap. This
causes zram to give a zero filled page that gets mapped to the
process, resulting in a user space crash later.

Consider parent and child processes Pa and Pb sharing the same swap
slot with swap_count 2. Swap is on zram with SWP_SYNCHRONOUS_IO set.
Virtual address 'VA' of Pa and Pb points to the shared swap entry.

Pa                                       Pb

fault on VA                              fault on VA
do_swap_page                             do_swap_page
lookup_swap_cache fails                  lookup_swap_cache fails
                                         Pb scheduled out
swapin_readahead (deletes zram entry)
swap_free (makes swap_count 1)
                                         Pb scheduled in
                                         swap_readpage (swap_count == 1)
                                         Takes SWP_SYNCHRONOUS_IO path
                                         zram enrty absent
                                         zram gives a zero filled page

Fix this by making sure that swap slot is freed only when swap count
drops down to one.

Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
Suggested-by: Minchan Kim <minchan@google.com>
Cc: Michal Hocko <mhocko@suse.com>
---
 mm/page_io.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/page_io.c b/mm/page_io.c
index 83db25a..3a198de 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -74,6 +74,7 @@ static void swap_slot_free_notify(struct page *page)
 {
 	struct swap_info_struct *sis;
 	struct gendisk *disk;
+	swp_entry_t entry;
 
 	/*
 	 * There is no guarantee that the page is in swap cache - the software
@@ -105,11 +106,10 @@ static void swap_slot_free_notify(struct page *page)
 	 * we again wish to reclaim it.
 	 */
 	disk = sis->bdev->bd_disk;
-	if (disk->fops->swap_slot_free_notify) {
-		swp_entry_t entry;
+	entry.val = page_private(page);
+	if (disk->fops->swap_slot_free_notify && __swap_count(entry) == 1) {
 		unsigned long offset;
 
-		entry.val = page_private(page);
 		offset = swp_offset(entry);
 
 		SetPageDirty(page);
-- 
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