Re: [BUGFIX][PATCH] fix corruption of hibernation caused by reusingswap at saving image

[KAMEZAWA Hiroyuki]

On Wed, 4 Aug 2010 11:31:22 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:

> On Wed, 4 Aug 2010 10:50:54 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> 
> > This patch is created against 2.6.35. CC'ed stable.
> > Thank you for all helps.
> > 
> 
> I'm sorry I now doubt this patch is wrong.

Changed the description.
Maybe the patch description/my understanding was wrong
but the patch itself should work.

==
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>

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 by the commit
c9e444103b5e7a5a3519f9913f59767f92e33baf.

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.

Here, memory is corrupted.


This patch is for avoiding bug by not reclaiming swap-entry at hibernation.
This is a quick fix for backporting.

Cc: stable@xxxxxxxxxx
Cc: Rafael J. Wysocki <rjw@xxxxxxx>
Reported-by: Ondreg Zary <linux@xxxxxxxxxxxxxxxxxxxx>
Tested-by: Andrea Gelmini <andrea.gelmini@xxxxxxxxx>
Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
---
 mm/swapfile.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

Index: linux-2.6.35.org/mm/swapfile.c
===================================================================
--- linux-2.6.35.org.orig/mm/swapfile.c
+++ linux-2.6.35.org/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);

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