Re: [PATCH v2 2/2] mm/ksm: fix advisor_min_pages_to_scan description

[Jinjiang Tu <tujinjiang@huawei.com>]

在 2026/8/13 22:32, Lorenzo Stoakes (ARM) 写道:
> Please show me the common courtesy of responding to _all_ the review given,
> not only the one point you want to respond to.
>
> I'm not happy with this patch, sorry, for the reasons stated that you chose
> to ignore.
>
> On Thu, Aug 13, 2026 at 07:17:05PM +0800, Jinjiang Tu wrote:
>> 在 2026/8/13 18:56, Lorenzo Stoakes (ARM) 写道:
>>> On Thu, Aug 13, 2026 at 11:17:22AM +0800, Jinjiang Tu wrote:
>>>> Both Documentation/admin-guide/mm/ksm.rst and the comment next to the
>>>> variable definition in mm/ksm.c describe advisor_min_pages_to_scan as a
>>>> lower limit of the pages_to_scan parameter, but that is not how the
>>>> scan-time advisor actually uses it. commit 4e5fa4f5eff6 ("mm/ksm: add ksm
>>>> advisor") only uses it to initialize ksm_thread_pages_to_scan when the
>>>> scan-time advisor is enabled. This will mislead the users.
>>>>
>>>> The semantics of advisor_min_pages_to_scan was updated in the v2 patchset
>>>> [1], but the documentation wasn't updated.
>>>>
>>>> Update the documentation and comment to match the semantics of
>>>> advisor_min_pages_to_scan.
>>> Umm, firstly if this were wrong you'd need to change the name of the variable
>>> too instad of documenting it as something completely distinct.
>>>
>>> But secondly AFAICT the logic in scan_time_advisor() suggests that it truly does
>>> act as a minimum bound as well as being the initial value for
>>> ksm_thread_pages_to_scan.
>> As the cover letter[1] said, 'The initial value and the max value for the pages_to_scan parameter can
>> be limited with:', ksm_advisor_min_pages is used as the initial value, not the min value of pages_to_scan.
> (Wrap your lines properly please, it's a common courtesy to learn how to
> send mail according to community conventions.)

Thanks for the reminder, I will follow this in the future.

> One of the part of my review you ignored:
>
> 	But in any case, you are making a claim here yet have provided no
> 	evidence for it. 'It is just the initial assignment' means nothing
> 	- if the logic can then only increase it up to the maximum value
> 	then it still acts as a minimum.

ksm_thread_pages_to_scan is adjusted by scan_time_advisor() after a full
scan finishes. ksm_thread_pages_to_scan could increase or decrease depend
on the real scan time is longer or shorter than the target scan time.
The min value of ksm_thread_pages_to_scan is only limited by
KSM_ADVISOR_MIN_CPU, so ksm_thread_pages_to_scan could be smaller
than ksm_advisor_min_pages_to_scan.

>
>> The v1 patchset[2] uses ksm_advisor_min_pages as the min value of pages_to_scan, but as David suggested, the
>> author changed the semantics of advisor_min_pages_to_scan in v2, but forgot to update the comments and variable names.
>>
>> [2] https://lore.kernel.org/all/20231004190249.829015-1-shr@devkernel.io/
>>
>>> So this patch just looks wrong to me.
>>>
>>> But in any case, you are making a claim here yet have provided no evidence for
>>> it. 'It is just the initial assignment' means nothing - if the logic can then
>>> only increase it up to the maximum value then it still acts as a minimum.
>>>
>>> A brief look at the logic suggests so.
>>>
>>> But in any case - the onus is on _you_ to prove that that's not happening.
>>>
>>> And even if you could do that, since it is _intended to be a lower bound_, the
>>> fix wouldn't be alterting comments or documentation, it would be to re-establish
>>> the minimum as a minimum.
>>>
>>>> Link: https://lore.kernel.org/linux-mm/20231028000945.2428830-2-shr@devkernel.io/ [1]
>>> This patch repeatedly states that it is the minimum time. You have failed to
>>> provide any analysis to suggest otherwise.
>>>
>>>> Signed-off-by: Jinjiang Tu <tujinjiang@huawei.com>
>>> Assisted-by: ?
>>>
>>>> ---
>>>>    Documentation/admin-guide/mm/ksm.rst | 4 ++--
>>>>    mm/ksm.c                             | 2 +-
>>>>    2 files changed, 3 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/Documentation/admin-guide/mm/ksm.rst b/Documentation/admin-guide/mm/ksm.rst
>>>> index c9f533b10f6f..c329ca747b8c 100644
>>>> --- a/Documentation/admin-guide/mm/ksm.rst
>>>> +++ b/Documentation/admin-guide/mm/ksm.rst
>>>> @@ -183,8 +183,8 @@ advisor_target_scan_time
>>>>            pages. The default value is 200 seconds.
>>>>
>>>>    advisor_min_pages_to_scan
>>>> -        specifies the lower limit of the ``pages_to_scan`` parameter of the
>>>> -        scan time advisor. The default is 500.
>>>> +        specifies the initial value of the ``pages_to_scan`` parameter of
>>>> +        the scan time advisor. The default is 500.
>>>>
>>>>    advisor_max_pages_to_scan
>>>>            specifies the upper limit of the ``pages_to_scan`` parameter of the
>>>> diff --git a/mm/ksm.c b/mm/ksm.c
>>>> index 7d5b76478f0b..4a6cf8cf5d60 100644
>>>> --- a/mm/ksm.c
>>>> +++ b/mm/ksm.c
>>>> @@ -342,7 +342,7 @@ static enum ksm_advisor_type ksm_advisor;
>>>>     * Only called through the sysfs control interface:
>>>>     */
>>>>
>>>> -/* At least scan this many pages per batch. */
>>>> +/* Initial number of pages to scan per batch. */
>>>>    static unsigned long ksm_advisor_min_pages_to_scan = 500;
>>>>
>>>>    static void set_advisor_defaults(void)
>>>> --
>>>> 2.43.0
>>>>
>>> --
>>> Cheers, Lorenzo
> --
> Cheers, Lorenzo
>