Re: [PATCH v2 1/3] mm/ksm: Fix incorrect KSM counter handling in mm_struct during fork

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

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.
>