mm: fix corruption of hibernation caused by reusing swap during image saving

Since 2.6.31, swap_map[]'s refcounting was changed to show that a used
swap entry is just for swap-cache, can be reused.  Then, while scanning
free entry in swap_map[], a swap entry may be able to be reclaimed and
reused.  It was caused by commit c9e444103b5e7a5 ("mm: reuse unused swap
entry if necessary").

But this caused deta corruption at resume. The scenario is

- Assume a clean-swap cache, but mapped.

- at hibernation_snapshot[], clean-swap-cache is saved as
  clean-swap-cache and swap_map[] is marked as SWAP_HAS_CACHE.

- then, save_image() is called.  And reuse SWAP_HAS_CACHE entry to save
  image, and break the contents.

After resume:

- the memory reclaim runs and finds clean-not-referenced-swap-cache and
  discards it because it's marked as clean.  But here, the contents on
  disk and swap-cache is inconsistent.

Hance memory is corrupted.

This patch avoids the bug by not reclaiming swap-entry during hibernation.
This is a quick fix for backporting.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Rafael J. Wysocki <rjw@sisk.pl>
Reported-by: Ondreg Zary <linux@rainbow-software.org>
Tested-by: Ondreg Zary <linux@rainbow-software.org>
Tested-by: Andrea Gelmini <andrea.gelmini@gmail.com>
Acked-by: Hugh Dickins <hughd@google.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 4 insertions, 2 deletions

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 03aa2d55f1a2..f08d165871b3 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -318,8 +318,10 @@ checks:
         if (offset > si->highest_bit)
                 scan_base = offset = si->lowest_bit;
 
-        /* reuse swap entry of cache-only swap if not busy. */
-        if (vm_swap_full() && si->swap_map[offset] == SWAP_HAS_CACHE) {
+        /* reuse swap entry of cache-only swap if not hibernation. */
+        if (vm_swap_full()
+                && usage == SWAP_HAS_CACHE
+                && si->swap_map[offset] == SWAP_HAS_CACHE) {
                 int swap_was_freed;
                 spin_unlock(&swap_lock);
                 swap_was_freed = __try_to_reclaim_swap(si, offset);