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Subject[PATCH 4.19 255/306] mm/page_io.c: do not free shared swap slots
From: Vinayak Menon <>

[ Upstream commit 5df373e95689b9519b8557da7c5bd0db0856d776 ]

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)
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.

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 <>
Signed-off-by: Sasha Levin <>
mm/page_io.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/mm/page_io.c b/mm/page_io.c
index aafd19ec1db46..08d2eae58fcee 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -76,6 +76,7 @@ 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
@@ -107,11 +108,11 @@ 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(sis, entry) == 1) {
unsigned long offset;

- entry.val = page_private(page);
offset = swp_offset(entry);


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