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 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)
On Mon, 15 Sep 2025 20:33:04 +0530 Donet Tom donettom@linux.ibm.com wrote:
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")
Linux-v5.19
Fixes: cb4df4cae4f2 ("ksm: count allocated ksm rmap_items for each process")
Linux-v6.1
Fixes: e2942062e01d ("ksm: count all zero pages placed by KSM")
Linux-v6.10
cc: stable@vger.kernel.org # v6.6
So how was Linux-v6.6 arrived at?
I think the most important use for Fixes: is to tell the -stable maintainers which kernel version(s) we believe should receive the patch. So listing multiple Fixes: targets just causes confusion.
Maybe the -stable maintainers parse the "# v6.6" thing, I don't know. But providing them with a single Fixes: recommendation is a good thing either way.
So can you please revisit this and suggest a single Fixes: target which clarifies your reccomendation?
Thanks.
(Cc Joe. Should checkpatch say something about this)?
On Mon, 2025-09-15 at 16:42 -0700, Andrew Morton wrote:
On Mon, 15 Sep 2025 20:33:04 +0530 Donet Tom donettom@linux.ibm.com wrote:
[]
Fixes: 7609385337a4 ("ksm: count ksm merging pages for each process")
Linux-v5.19
Fixes: cb4df4cae4f2 ("ksm: count allocated ksm rmap_items for each process")
Linux-v6.1
Fixes: e2942062e01d ("ksm: count all zero pages placed by KSM")
Linux-v6.10
cc: stable@vger.kernel.org # v6.6
So how was Linux-v6.6 arrived at?
[]
(Cc Joe. Should checkpatch say something about this)?
Probably not. Parsing variants of versions seems, umm, difficult.
On Mon, 15 Sep 2025 19:14:47 -0700 Joe Perches joe@perches.com wrote:
On Mon, 2025-09-15 at 16:42 -0700, Andrew Morton wrote:
On Mon, 15 Sep 2025 20:33:04 +0530 Donet Tom donettom@linux.ibm.com wrote:
[]
Fixes: 7609385337a4 ("ksm: count ksm merging pages for each process")
Linux-v5.19
Fixes: cb4df4cae4f2 ("ksm: count allocated ksm rmap_items for each process")
Linux-v6.1
Fixes: e2942062e01d ("ksm: count all zero pages placed by KSM")
Linux-v6.10
cc: stable@vger.kernel.org # v6.6
So how was Linux-v6.6 arrived at?
[]
(Cc Joe. Should checkpatch say something about this)?
Probably not. Parsing variants of versions seems, umm, difficult.
I was thinking simpler. If the Fixes: count exceeds one, ask "wtf are you asking of the -stable maintainers"?
On Mon, Sep 15, 2025 at 04:42:48PM -0700, Andrew Morton wrote:
On Mon, 15 Sep 2025 20:33:04 +0530 Donet Tom donettom@linux.ibm.com wrote:
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")
Linux-v5.19
Fixes: cb4df4cae4f2 ("ksm: count allocated ksm rmap_items for each process")
Linux-v6.1
Fixes: e2942062e01d ("ksm: count all zero pages placed by KSM")
Linux-v6.10
cc: stable@vger.kernel.org # v6.6
So how was Linux-v6.6 arrived at?
e2942062e01d is in v6.6, not in v6.10 - I suspect that this is why the "# v6.6" part was added.
I think the most important use for Fixes: is to tell the -stable maintainers which kernel version(s) we believe should receive the patch. So listing multiple Fixes: targets just causes confusion.
Right - there's no way of communicating if all the commits listed in multiple Fixes tags should exist in the tree, or any one of them, for the new fix to be applicable.
On Tue, 16 Sep 2025 00:33:09 -0400 Sasha Levin sashal@kernel.org wrote:
On Mon, Sep 15, 2025 at 04:42:48PM -0700, Andrew Morton wrote:
On Mon, 15 Sep 2025 20:33:04 +0530 Donet Tom donettom@linux.ibm.com wrote:
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")
Linux-v5.19
Fixes: cb4df4cae4f2 ("ksm: count allocated ksm rmap_items for each process")
Linux-v6.1
Fixes: e2942062e01d ("ksm: count all zero pages placed by KSM")
Linux-v6.10
cc: stable@vger.kernel.org # v6.6
So how was Linux-v6.6 arrived at?
e2942062e01d is in v6.6, not in v6.10 - I suspect that this is why the "# v6.6" part was added.
OK.
I think the most important use for Fixes: is to tell the -stable maintainers which kernel version(s) we believe should receive the patch. So listing multiple Fixes: targets just causes confusion.
Right - there's no way of communicating if all the commits listed in multiple Fixes tags should exist in the tree, or any one of them, for the new fix to be applicable.
So what should we do in this situation?
On Mon, Sep 15, 2025 at 09:41:17PM -0700, Andrew Morton wrote:
On Tue, 16 Sep 2025 00:33:09 -0400 Sasha Levin sashal@kernel.org wrote:
On Mon, Sep 15, 2025 at 04:42:48PM -0700, Andrew Morton wrote:
I think the most important use for Fixes: is to tell the -stable maintainers which kernel version(s) we believe should receive the patch. So listing multiple Fixes: targets just causes confusion.
Right - there's no way of communicating if all the commits listed in multiple Fixes tags should exist in the tree, or any one of them, for the new fix to be applicable.
So what should we do in this situation?
For us, ideally point to the oldest commit fixed by the new commit with a Fixes: tag, and then a note along with the stable tag if this is a more complex scenario that we need to consider.
On 9/16/25 10:03 AM, Sasha Levin wrote:
On Mon, Sep 15, 2025 at 04:42:48PM -0700, Andrew Morton wrote:
On Mon, 15 Sep 2025 20:33:04 +0530 Donet Tom donettom@linux.ibm.com wrote:
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")
Linux-v5.19
Fixes: cb4df4cae4f2 ("ksm: count allocated ksm rmap_items for each process")
Linux-v6.1
Fixes: e2942062e01d ("ksm: count all zero pages placed by KSM")
Linux-v6.10
cc: stable@vger.kernel.org # v6.6
So how was Linux-v6.6 arrived at?
e2942062e01d is in v6.6, not in v6.10 - I suspect that this is why the "# v6.6" part was added.
Yes, e2942062e01d is in v6.6, which is why I mentioned that we need to backport it up to v6.6.
I think the most important use for Fixes: is to tell the -stable maintainers which kernel version(s) we believe should receive the patch. So listing multiple Fixes: targets just causes confusion.
Right - there's no way of communicating if all the commits listed in multiple Fixes tags should exist in the tree, or any one of them, for the new fix to be applicable.
linux-stable-mirror@lists.linaro.org
-
Andrew Morton -
Donet Tom -
Joe Perches -
Sasha Levin