Commit 5df373e9 authored by Vinayak Menon's avatar Vinayak Menon Committed by Linus Torvalds
Browse files

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.

Fixes: aa8d22a1

 ("mm: swap: SWP_SYNCHRONOUS_IO: skip swapcache only if swapped page has no other reference")
Signed-off-by: default avatarVinayak Menon <>
Suggested-by: default avatarMinchan Kim <>
Acked-by: default avatarMinchan Kim <>
Cc: Michal Hocko <>
Cc: Hugh Dickins <>
Cc: <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent 2c91f8fc
......@@ -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);
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