The MADV_FREE patchset changes page reclaim to simply free a clean
anonymous page with no dirty ptes, instead of swapping it out; but
KSM uses clean write-protected ptes to reference the stable ksm page.
So be sure to mark that page dirty, so it's never mistakenly discarded.

[hughd: adjusted comments]
Acked-by: Hugh Dickins <hu...@google.com>
Signed-off-by: Minchan Kim <minc...@kernel.org>
---
 mm/ksm.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/ksm.c b/mm/ksm.c
index 7ee101eaacdf..18d2b7afecff 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1053,6 +1053,12 @@ static int try_to_merge_one_page(struct vm_area_struct 
*vma,
                         */
                        set_page_stable_node(page, NULL);
                        mark_page_accessed(page);
+                       /*
+                        * Page reclaim just frees a clean page with no dirty
+                        * ptes: make sure that the ksm page would be swapped.
+                        */
+                       if (!PageDirty(page))
+                               SetPageDirty(page);
                        err = 0;
                } else if (pages_identical(page, kpage))
                        err = replace_page(vma, page, kpage, orig_pte);
-- 
1.9.1

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