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Subject[031/103] ksm: fix bad user data when swapping
2.6.35-stable review patch.  If anyone has any objections, please let us know.


From: Hugh Dickins <>

commit 4e31635c367a9e21a43cfbfae4c9deda2e19d1f4 upstream.

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 <>
Cc: Andrew Morton <>
Cc: Andrea Arcangeli <>
Signed-off-by: Linus Torvalds <>
Signed-off-by: Greg Kroah-Hartman <>

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

--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -731,7 +731,7 @@ 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);
@@ -754,7 +754,9 @@ 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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