diff options
authorVinayak Menon <>2019-11-15 17:35:00 -0800
committerLinus Torvalds <>2019-11-15 18:34:00 -0800
commit5df373e95689b9519b8557da7c5bd0db0856d776 (patch)
parent2c91f8fc6c999fe10185d8ad99fda1759f662f70 (diff)
mm/page_io.c: do not free shared swap slots
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. Link: Fixes: aa8d22a11da9 ("mm: swap: SWP_SYNCHRONOUS_IO: skip swapcache only if swapped page has no other reference") Signed-off-by: Vinayak Menon <> Suggested-by: Minchan Kim <> Acked-by: Minchan Kim <> Cc: Michal Hocko <> Cc: Hugh Dickins <> Cc: <> Signed-off-by: Andrew Morton <> Signed-off-by: Linus Torvalds <>
1 files changed, 3 insertions, 3 deletions
diff --git a/mm/page_io.c b/mm/page_io.c
index 24ee600f9131..60a66a58b9bf 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -73,6 +73,7 @@ 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
@@ -104,11 +105,10 @@ 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);