[PATCH] ksm: fix bad user data when swapping

[Hugh Dickins <hughd@google.com>]

Building under memory pressure, with KSM on 2.6.36-rc5, collapsed with
an internal compiler error: typically indicating an error in swapping.

Perhaps there's a timing issue which makes it now more likely, perhaps
it's just a long time since I tried for so long: this bug goes back to
KSM swapping in 2.6.33.

Notice how reuse_swap_page() allows an exclusive page to be reused, but
only does SetPageDirty if it can delete it from swap cache right then -
if it's currently under Writeback, it has to be left in cache and we
don't SetPageDirty, but the page can be reused.  Fine, the dirty bit
will get set in the pte; but notice how zap_pte_range() does not bother
to transfer pte_dirty to page_dirty when unmapping a PageAnon.

If KSM chooses to share such a page, it will look like a clean copy of
swapcache, and not be written out to swap when its memory is needed;
then stale data read back from swap when it's needed again.

We could fix this in reuse_swap_page() (or even refuse to reuse a
page under writeback), but it's more honest to fix my oversight in
KSM's write_protect_page().  Several days of testing on three machines
confirms that this fixes the issue they showed.

Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: stable@kernel.org
---

 mm/ksm.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- 2.6.36-rc6/mm/ksm.c	2010-09-12 17:34:03.000000000 -0700
+++ linux/mm/ksm.c	2010-09-28 23:27:05.000000000 -0700
@@ -712,7 +712,7 @@ static int write_protect_page(struct vm_
 	if (!ptep)
 		goto out;
 
-	if (pte_write(*ptep)) {
+	if (pte_write(*ptep) || pte_dirty(*ptep)) {
 		pte_t entry;
 
 		swapped = PageSwapCache(page);
@@ -735,7 +735,9 @@ static int write_protect_page(struct vm_
 			set_pte_at(mm, addr, ptep, entry);
 			goto out_unlock;
 		}
-		entry = pte_wrprotect(entry);
+		if (pte_dirty(entry))
+			set_page_dirty(page);
+		entry = pte_mkclean(pte_wrprotect(entry));
 		set_pte_at_notify(mm, addr, ptep, entry);
 	}
 	*orig_pte = *ptep;

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