[PATCH v3 1/2] mm/ksm: Fix incorrect KSM counter handling in mm_struct during fork

[Donet Tom <donettom@linux.ibm.com>]

Currently, the KSM-related counters in `mm_struct`, such as
`ksm_merging_pages`, `ksm_rmap_items`, and `ksm_zero_pages`, are
inherited by the child process during fork. This results in inconsistent
accounting.

When a process uses KSM, identical pages are merged and an rmap item is
created for each merged page. The `ksm_merging_pages` and
`ksm_rmap_items` counters are updated accordingly. However, after a
fork, these counters are copied to the child while the corresponding
rmap items are not. As a result, when the child later triggers an
unmerge, there are no rmap items present in the child, so the counters
remain stale, leading to incorrect accounting.

A similar issue exists with `ksm_zero_pages`, which maintains both a
global counter and a per-process counter. During fork, the per-process
counter is inherited by the child, but the global counter is not
incremented. Since the child also references zero pages, the global
counter should be updated as well. Otherwise, during zero-page unmerge,
both the global and per-process counters are decremented, causing the
global counter to become inconsistent.

To fix this, ksm_merging_pages and ksm_rmap_items are reset to 0
during fork, and the global ksm_zero_pages counter is updated with the
per-process ksm_zero_pages value inherited by the child. This ensures
that KSM statistics remain accurate and reflect the activity of each
process correctly.

Fixes: 7609385337a4 ("ksm: count ksm merging pages for each process")
Fixes: cb4df4cae4f2 ("ksm: count allocated ksm rmap_items for each process")
Fixes: e2942062e01d ("ksm: count all zero pages placed by KSM")
cc: stable@vger.kernel.org # v6.6
Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Donet Tom <donettom@linux.ibm.com>
---
 include/linux/ksm.h | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/include/linux/ksm.h b/include/linux/ksm.h
index 22e67ca7cba3..067538fc4d58 100644
--- a/include/linux/ksm.h
+++ b/include/linux/ksm.h
@@ -56,8 +56,14 @@ static inline long mm_ksm_zero_pages(struct mm_struct *mm)
 static inline void ksm_fork(struct mm_struct *mm, struct mm_struct *oldmm)
 {
 	/* Adding mm to ksm is best effort on fork. */
-	if (mm_flags_test(MMF_VM_MERGEABLE, oldmm))
+	if (mm_flags_test(MMF_VM_MERGEABLE, oldmm)) {
+		long nr_ksm_zero_pages = atomic_long_read(&mm->ksm_zero_pages);
+
+		mm->ksm_merging_pages = 0;
+		mm->ksm_rmap_items = 0;
+		atomic_long_add(nr_ksm_zero_pages, &ksm_zero_pages);
 		__ksm_enter(mm);
+	}
 }
 
 static inline int ksm_execve(struct mm_struct *mm)
-- 
2.51.0