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@xxxxxxxxxxxxxx>
Suggested-by: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
---
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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