The patch titled Subject: mm/ksm: fix incorrect KSM counter handling in mm_struct during fork has been added to the -mm mm-new branch. Its filename is mm-ksm-fix-incorrect-ksm-counter-handling-in-mm_struct-during-fork.patch
This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...
This patch will later appear in the mm-new branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------ From: Donet Tom donettom@linux.ibm.com Subject: mm/ksm: fix incorrect KSM counter handling in mm_struct during fork Date: Mon, 15 Sep 2025 20:33:04 +0530
Patch series "mm/ksm: Fix incorrect accounting of KSM counters during fork.", v2.
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 two patches add selftests to verify that the ksm_merging_pages counter and the global ksm_zero_pages counter are 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 # [RUN] test_fork_global_ksm_zero_pages_count not ok 11 Incorrect global ksm zero page counter after fork
With the first patch -------------------- # [RUN] test_fork_ksm_merging_page_count ok 10 ksm_merging_pages is not inherited after fork # [RUN] test_fork_global_ksm_zero_pages_count ok 11 Global ksm zero page count is correct after fork
This patch (of 3):
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.
Link: https://lkml.kernel.org/r/cover.1757946863.git.donettom@linux.ibm.com Link: https://lkml.kernel.org/r/4044e7623953d9f4c240d0308cf0b2fe769ee553.175794686... 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") Signed-off-by: Donet Tom donettom@linux.ibm.com Cc: Aboorva Devarajan aboorvad@linux.ibm.com Cc: Chengming Zhou chengming.zhou@linux.dev Cc: David Hildenbrand david@redhat.com Cc: "Ritesh Harjani (IBM)" ritesh.list@gmail.com Cc: Wei Yang richard.weiyang@gmail.com Cc: xu xin xu.xin16@zte.com.cn Cc: stable@vger.kernel.org [6.6] Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
include/linux/ksm.h | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-)
--- a/include/linux/ksm.h~mm-ksm-fix-incorrect-ksm-counter-handling-in-mm_struct-during-fork +++ a/include/linux/ksm.h @@ -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 donettom@linux.ibm.com are
mm-ksm-fix-incorrect-ksm-counter-handling-in-mm_struct-during-fork.patch selftests-mm-added-fork-inheritance-test-for-ksm_merging_pages-counter.patch selftests-mm-added-fork-test-to-verify-global-ksm_zero_pages-counter-behavior.patch