mm-ksm-fix-incorrect-ksm-counter-handling-in-mm_struct-during-fork.patch removed from -mm tree

The quilt patch titled
Subject: mm/ksm: fix incorrect KSM counter handling in mm_struct during fork
has been removed from the -mm tree. Its filename was
mm-ksm-fix-incorrect-ksm-counter-handling-in-mm_struct-during-fork.patch

This patch was dropped because it was merged into the mm-stable branch of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

------------------------------------------------------
From: Donet Tom <[email protected]>
Subject: mm/ksm: fix incorrect KSM counter handling in mm_struct during fork
Date: Wed, 24 Sep 2025 00:16:59 +0530

Patch series "mm/ksm: Fix incorrect accounting of KSM counters during fork", v3.

The first patch in this series fixes the incorrect accounting of KSM counters such as ksm_merging_pages, ksm_rmap_items, and the global ksm_zero_pages during fork.

The following patch add a selftest to verify the ksm_merging_pages counter was updated correctly during fork.

Test Results
============
Without the first patch
-----------------------
# [RUN] test_fork_ksm_merging_page_count
not ok 10 ksm_merging_page in child: 32

With the first patch
--------------------
# [RUN] test_fork_ksm_merging_page_count
ok 10 ksm_merging_pages is not inherited after fork

This patch (of 2):

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.

---

 include/linux/ksm.h |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

@@ -56,8 +56,14 @@ static inline long mm_ksm_zero_pages(str
 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)

_

Patches currently in -mm which might be from [email protected] are