[PATCH] mm: huge_memory: don't start_stop_khugepaged for non-PMD THP

[PATCH] mm: huge_memory: don't start_stop_khugepaged for non-PMD THP

[Barry Song]

From: Barry Song <v-songbaohua@oppo.com>

khugepaged will be automatically started when PMD-sized THP is enabled
(either of the per-size anon control or the top-level control are set
to "always" or "madvise"), and it'll be automatically shutdown when
PMD-sized THP is disabled (when both the per-size anon control and the
top-level control are "never").

It seems unnecessary to call start_stop_khugepaged() for non-PMD THP,
as it would only waste CPU time.

Cc: Lance Yang <ioworker0@gmail.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Barry Song <v-songbaohua@oppo.com>
---
 mm/huge_memory.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 41460847988c..bd365e35acf7 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -514,7 +514,7 @@ static ssize_t thpsize_enabled_store(struct kobject *kobj,
 	} else
 		ret = -EINVAL;
 
-	if (ret > 0) {
+	if (ret > 0 && order == HPAGE_PMD_ORDER) {
 		int err;
 
 		err = start_stop_khugepaged();
-- 
2.34.1

Re: [PATCH] mm: huge_memory: don't start_stop_khugepaged for non-PMD THP

[Ryan Roberts]

On 26/07/2024 09:28, Barry Song wrote:
> From: Barry Song <v-songbaohua@oppo.com>
> 
> khugepaged will be automatically started when PMD-sized THP is enabled
> (either of the per-size anon control or the top-level control are set
> to "always" or "madvise"), and it'll be automatically shutdown when
> PMD-sized THP is disabled (when both the per-size anon control and the
> top-level control are "never").
> 
> It seems unnecessary to call start_stop_khugepaged() for non-PMD THP,
> as it would only waste CPU time.
> 
> Cc: Lance Yang <ioworker0@gmail.com>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Yang Shi <shy828301@gmail.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> ---
>  mm/huge_memory.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 41460847988c..bd365e35acf7 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -514,7 +514,7 @@ static ssize_t thpsize_enabled_store(struct kobject *kobj,
>  	} else
>  		ret = -EINVAL;
>  
> -	if (ret > 0) {
> +	if (ret > 0 && order == HPAGE_PMD_ORDER) {
>  		int err;
>  
>  		err = start_stop_khugepaged();

Personally I see this as a bit of a layering violation; its
start_stop_khugepaged() that should decide the policy for when to start and stop
the daemon. thpsize_enabled_store() should just be calling
start_stop_khugepaged() to notify that something potentially pertinent to the a
policy decision has changed.

And I don't think this should be a hot path; I'd expect the global thp size
configuration to be set at boot and remain pretty static. Do you have evidence
to the contrary?

Re: [PATCH] mm: huge_memory: don't start_stop_khugepaged for non-PMD THP

[David Hildenbrand]

On 26.07.24 11:43, Ryan Roberts wrote:
> On 26/07/2024 09:28, Barry Song wrote:
>> From: Barry Song <v-songbaohua@oppo.com>
>>
>> khugepaged will be automatically started when PMD-sized THP is enabled
>> (either of the per-size anon control or the top-level control are set
>> to "always" or "madvise"), and it'll be automatically shutdown when
>> PMD-sized THP is disabled (when both the per-size anon control and the
>> top-level control are "never").
>>
>> It seems unnecessary to call start_stop_khugepaged() for non-PMD THP,
>> as it would only waste CPU time.
>>
>> Cc: Lance Yang <ioworker0@gmail.com>
>> Cc: Ryan Roberts <ryan.roberts@arm.com>
>> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
>> Cc: David Hildenbrand <david@redhat.com>
>> Cc: Yang Shi <shy828301@gmail.com>
>> Cc: Zi Yan <ziy@nvidia.com>
>> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
>> ---
>>   mm/huge_memory.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 41460847988c..bd365e35acf7 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -514,7 +514,7 @@ static ssize_t thpsize_enabled_store(struct kobject *kobj,
>>   	} else
>>   		ret = -EINVAL;
>>   
>> -	if (ret > 0) {
>> +	if (ret > 0 && order == HPAGE_PMD_ORDER) {
>>   		int err;
>>   
>>   		err = start_stop_khugepaged();
> 
> Personally I see this as a bit of a layering violation; its
> start_stop_khugepaged() that should decide the policy for when to start and stop
> the daemon. thpsize_enabled_store() should just be calling
> start_stop_khugepaged() to notify that something potentially pertinent to the a
> policy decision has changed.

Agreed, skimming the subject I was under the impression that we would be 
fixing something here.

-- 
Cheers,

David / dhildenb

Re: [PATCH] mm: huge_memory: don't start_stop_khugepaged for non-PMD THP

[Barry Song]

On Fri, Jul 26, 2024 at 9:48 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 26.07.24 11:43, Ryan Roberts wrote:
> > On 26/07/2024 09:28, Barry Song wrote:
> >> From: Barry Song <v-songbaohua@oppo.com>
> >>
> >> khugepaged will be automatically started when PMD-sized THP is enabled
> >> (either of the per-size anon control or the top-level control are set
> >> to "always" or "madvise"), and it'll be automatically shutdown when
> >> PMD-sized THP is disabled (when both the per-size anon control and the
> >> top-level control are "never").
> >>
> >> It seems unnecessary to call start_stop_khugepaged() for non-PMD THP,
> >> as it would only waste CPU time.
> >>
> >> Cc: Lance Yang <ioworker0@gmail.com>
> >> Cc: Ryan Roberts <ryan.roberts@arm.com>
> >> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> >> Cc: David Hildenbrand <david@redhat.com>
> >> Cc: Yang Shi <shy828301@gmail.com>
> >> Cc: Zi Yan <ziy@nvidia.com>
> >> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> >> ---
> >>   mm/huge_memory.c | 2 +-
> >>   1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> >> index 41460847988c..bd365e35acf7 100644
> >> --- a/mm/huge_memory.c
> >> +++ b/mm/huge_memory.c
> >> @@ -514,7 +514,7 @@ static ssize_t thpsize_enabled_store(struct kobject *kobj,
> >>      } else
> >>              ret = -EINVAL;
> >>
> >> -    if (ret > 0) {
> >> +    if (ret > 0 && order == HPAGE_PMD_ORDER) {
> >>              int err;
> >>
> >>              err = start_stop_khugepaged();
> >
> > Personally I see this as a bit of a layering violation; its
> > start_stop_khugepaged() that should decide the policy for when to start and stop
> > the daemon. thpsize_enabled_store() should just be calling
> > start_stop_khugepaged() to notify that something potentially pertinent to the a
> > policy decision has changed.
>

My impression is that it slightly deviates from the huge page documentation in
Documentation/admin-guide/mm/transhuge.rst.

khugepaged will be automatically started when PMD-sized THP is enabled
(either of the per-size anon control or the top-level control are set
to "always" or "madvise"), and it'll be automatically shutdown when
PMD-sized THP is disabled (when both the per-size anon control and the
top-level control are "never").

non-PMD size is not involved in khugepaged, but I agree the policy might change
in the future.

> Agreed, skimming the subject I was under the impression that we would be
> fixing something here.

working on another swapin_enabled and reviewing the enabled source code.
I don't need this startstop for all sizes in that case, so I made a
quick adjustment
to this part as well. If neither of you likes it, that's fine with me :-)

>
> --
> Cheers,
>
> David / dhildenb
>

Thanks
Barry

Re: [PATCH] mm: huge_memory: don't start_stop_khugepaged for non-PMD THP

[Ryan Roberts]

On 26/07/2024 11:04, Barry Song wrote:
> On Fri, Jul 26, 2024 at 9:48 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 26.07.24 11:43, Ryan Roberts wrote:
>>> On 26/07/2024 09:28, Barry Song wrote:
>>>> From: Barry Song <v-songbaohua@oppo.com>
>>>>
>>>> khugepaged will be automatically started when PMD-sized THP is enabled
>>>> (either of the per-size anon control or the top-level control are set
>>>> to "always" or "madvise"), and it'll be automatically shutdown when
>>>> PMD-sized THP is disabled (when both the per-size anon control and the
>>>> top-level control are "never").
>>>>
>>>> It seems unnecessary to call start_stop_khugepaged() for non-PMD THP,
>>>> as it would only waste CPU time.
>>>>
>>>> Cc: Lance Yang <ioworker0@gmail.com>
>>>> Cc: Ryan Roberts <ryan.roberts@arm.com>
>>>> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
>>>> Cc: David Hildenbrand <david@redhat.com>
>>>> Cc: Yang Shi <shy828301@gmail.com>
>>>> Cc: Zi Yan <ziy@nvidia.com>
>>>> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
>>>> ---
>>>>   mm/huge_memory.c | 2 +-
>>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>> index 41460847988c..bd365e35acf7 100644
>>>> --- a/mm/huge_memory.c
>>>> +++ b/mm/huge_memory.c
>>>> @@ -514,7 +514,7 @@ static ssize_t thpsize_enabled_store(struct kobject *kobj,
>>>>      } else
>>>>              ret = -EINVAL;
>>>>
>>>> -    if (ret > 0) {
>>>> +    if (ret > 0 && order == HPAGE_PMD_ORDER) {
>>>>              int err;
>>>>
>>>>              err = start_stop_khugepaged();
>>>
>>> Personally I see this as a bit of a layering violation; its
>>> start_stop_khugepaged() that should decide the policy for when to start and stop
>>> the daemon. thpsize_enabled_store() should just be calling
>>> start_stop_khugepaged() to notify that something potentially pertinent to the a
>>> policy decision has changed.
>>
> 
> My impression is that it slightly deviates from the huge page documentation in
> Documentation/admin-guide/mm/transhuge.rst.
> 
> khugepaged will be automatically started when PMD-sized THP is enabled
> (either of the per-size anon control or the top-level control are set
> to "always" or "madvise"), and it'll be automatically shutdown when
> PMD-sized THP is disabled (when both the per-size anon control and the
> top-level control are "never").

But start_stop_khugepaged() doesn't unconditionally start khugepaged, it takes
action based on hugepage_pmd_enabled() which only returns true if there are any
pmd sized THP enabled (currently looking at anon and file, but should also look
at shmem in future; that's a known bug that's been there forever). So I don't
think it is inconsistent with the documentation?

> 
> non-PMD size is not involved in khugepaged, but I agree the policy might change
> in the future.
> 
>> Agreed, skimming the subject I was under the impression that we would be
>> fixing something here.
> 
> working on another swapin_enabled and reviewing the enabled source code.
> I don't need this startstop for all sizes in that case, so I made a
> quick adjustment
> to this part as well. If neither of you likes it, that's fine with me :-)
> 
>>
>> --
>> Cheers,
>>
>> David / dhildenb
>>
> 
> Thanks
> Barry

Re: [PATCH] mm: huge_memory: don't start_stop_khugepaged for non-PMD THP

[Barry Song]

On Fri, Jul 26, 2024 at 10:45 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>
> On 26/07/2024 11:04, Barry Song wrote:
> > On Fri, Jul 26, 2024 at 9:48 PM David Hildenbrand <david@redhat.com> wrote:
> >>
> >> On 26.07.24 11:43, Ryan Roberts wrote:
> >>> On 26/07/2024 09:28, Barry Song wrote:
> >>>> From: Barry Song <v-songbaohua@oppo.com>
> >>>>
> >>>> khugepaged will be automatically started when PMD-sized THP is enabled
> >>>> (either of the per-size anon control or the top-level control are set
> >>>> to "always" or "madvise"), and it'll be automatically shutdown when
> >>>> PMD-sized THP is disabled (when both the per-size anon control and the
> >>>> top-level control are "never").
> >>>>
> >>>> It seems unnecessary to call start_stop_khugepaged() for non-PMD THP,
> >>>> as it would only waste CPU time.
> >>>>
> >>>> Cc: Lance Yang <ioworker0@gmail.com>
> >>>> Cc: Ryan Roberts <ryan.roberts@arm.com>
> >>>> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> >>>> Cc: David Hildenbrand <david@redhat.com>
> >>>> Cc: Yang Shi <shy828301@gmail.com>
> >>>> Cc: Zi Yan <ziy@nvidia.com>
> >>>> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> >>>> ---
> >>>>   mm/huge_memory.c | 2 +-
> >>>>   1 file changed, 1 insertion(+), 1 deletion(-)
> >>>>
> >>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> >>>> index 41460847988c..bd365e35acf7 100644
> >>>> --- a/mm/huge_memory.c
> >>>> +++ b/mm/huge_memory.c
> >>>> @@ -514,7 +514,7 @@ static ssize_t thpsize_enabled_store(struct kobject *kobj,
> >>>>      } else
> >>>>              ret = -EINVAL;
> >>>>
> >>>> -    if (ret > 0) {
> >>>> +    if (ret > 0 && order == HPAGE_PMD_ORDER) {
> >>>>              int err;
> >>>>
> >>>>              err = start_stop_khugepaged();
> >>>
> >>> Personally I see this as a bit of a layering violation; its
> >>> start_stop_khugepaged() that should decide the policy for when to start and stop
> >>> the daemon. thpsize_enabled_store() should just be calling
> >>> start_stop_khugepaged() to notify that something potentially pertinent to the a
> >>> policy decision has changed.
> >>
> >
> > My impression is that it slightly deviates from the huge page documentation in
> > Documentation/admin-guide/mm/transhuge.rst.
> >
> > khugepaged will be automatically started when PMD-sized THP is enabled
> > (either of the per-size anon control or the top-level control are set
> > to "always" or "madvise"), and it'll be automatically shutdown when
> > PMD-sized THP is disabled (when both the per-size anon control and the
> > top-level control are "never").
>
> But start_stop_khugepaged() doesn't unconditionally start khugepaged, it takes
> action based on hugepage_pmd_enabled() which only returns true if there are any
> pmd sized THP enabled (currently looking at anon and file, but should also look
> at shmem in future; that's a known bug that's been there forever). So I don't
> think it is inconsistent with the documentation?

My point is that, as a code reader, if non-PMD sizes are never
involved with khugepaged,
we can proceed without needing to check the lower-layer code.
otherwise it is a bit
confusing, especially since start_stop_khugepaged() unconditionally calls
set_recommended_min_free_kbytes() for all sizes, regardless of whether
they are PMD.

However, I agree with your point that if non-PMD sizes are eventually managed by
khugepaged, then this approach is flawed. This is the downside of this change.

>
> >
> > non-PMD size is not involved in khugepaged, but I agree the policy might change
> > in the future.
> >
> >> Agreed, skimming the subject I was under the impression that we would be
> >> fixing something here.
> >
> > working on another swapin_enabled and reviewing the enabled source code.
> > I don't need this startstop for all sizes in that case, so I made a
> > quick adjustment
> > to this part as well. If neither of you likes it, that's fine with me :-)
> >
> >>
> >> --
> >> Cheers,
> >>
> >> David / dhildenb
> >>

Thanks
Barry

Re: [PATCH] mm: huge_memory: don't start_stop_khugepaged for non-PMD THP

[Ryan Roberts]

On 26/07/2024 12:00, Barry Song wrote:
> On Fri, Jul 26, 2024 at 10:45 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>
>> On 26/07/2024 11:04, Barry Song wrote:
>>> On Fri, Jul 26, 2024 at 9:48 PM David Hildenbrand <david@redhat.com> wrote:
>>>>
>>>> On 26.07.24 11:43, Ryan Roberts wrote:
>>>>> On 26/07/2024 09:28, Barry Song wrote:
>>>>>> From: Barry Song <v-songbaohua@oppo.com>
>>>>>>
>>>>>> khugepaged will be automatically started when PMD-sized THP is enabled
>>>>>> (either of the per-size anon control or the top-level control are set
>>>>>> to "always" or "madvise"), and it'll be automatically shutdown when
>>>>>> PMD-sized THP is disabled (when both the per-size anon control and the
>>>>>> top-level control are "never").
>>>>>>
>>>>>> It seems unnecessary to call start_stop_khugepaged() for non-PMD THP,
>>>>>> as it would only waste CPU time.
>>>>>>
>>>>>> Cc: Lance Yang <ioworker0@gmail.com>
>>>>>> Cc: Ryan Roberts <ryan.roberts@arm.com>
>>>>>> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
>>>>>> Cc: David Hildenbrand <david@redhat.com>
>>>>>> Cc: Yang Shi <shy828301@gmail.com>
>>>>>> Cc: Zi Yan <ziy@nvidia.com>
>>>>>> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
>>>>>> ---
>>>>>>   mm/huge_memory.c | 2 +-
>>>>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>>>> index 41460847988c..bd365e35acf7 100644
>>>>>> --- a/mm/huge_memory.c
>>>>>> +++ b/mm/huge_memory.c
>>>>>> @@ -514,7 +514,7 @@ static ssize_t thpsize_enabled_store(struct kobject *kobj,
>>>>>>      } else
>>>>>>              ret = -EINVAL;
>>>>>>
>>>>>> -    if (ret > 0) {
>>>>>> +    if (ret > 0 && order == HPAGE_PMD_ORDER) {
>>>>>>              int err;
>>>>>>
>>>>>>              err = start_stop_khugepaged();
>>>>>
>>>>> Personally I see this as a bit of a layering violation; its
>>>>> start_stop_khugepaged() that should decide the policy for when to start and stop
>>>>> the daemon. thpsize_enabled_store() should just be calling
>>>>> start_stop_khugepaged() to notify that something potentially pertinent to the a
>>>>> policy decision has changed.
>>>>
>>>
>>> My impression is that it slightly deviates from the huge page documentation in
>>> Documentation/admin-guide/mm/transhuge.rst.
>>>
>>> khugepaged will be automatically started when PMD-sized THP is enabled
>>> (either of the per-size anon control or the top-level control are set
>>> to "always" or "madvise"), and it'll be automatically shutdown when
>>> PMD-sized THP is disabled (when both the per-size anon control and the
>>> top-level control are "never").
>>
>> But start_stop_khugepaged() doesn't unconditionally start khugepaged, it takes
>> action based on hugepage_pmd_enabled() which only returns true if there are any
>> pmd sized THP enabled (currently looking at anon and file, but should also look
>> at shmem in future; that's a known bug that's been there forever). So I don't
>> think it is inconsistent with the documentation?
> 
> My point is that, as a code reader, if non-PMD sizes are never
> involved with khugepaged,
> we can proceed without needing to check the lower-layer code.
> otherwise it is a bit
> confusing, especially since start_stop_khugepaged() unconditionally calls
> set_recommended_min_free_kbytes() for all sizes, regardless of whether
> they are PMD.
> 
> However, I agree with your point that if non-PMD sizes are eventually managed by
> khugepaged, then this approach is flawed. This is the downside of this change.

Yes, I'd hope that eventually khugepaged will support non-PMD sizes. That's
something we are hoping to look at later this year. So my personal preference is
to leave as is.

> 
>>
>>>
>>> non-PMD size is not involved in khugepaged, but I agree the policy might change
>>> in the future.
>>>
>>>> Agreed, skimming the subject I was under the impression that we would be
>>>> fixing something here.
>>>
>>> working on another swapin_enabled and reviewing the enabled source code.
>>> I don't need this startstop for all sizes in that case, so I made a
>>> quick adjustment
>>> to this part as well. If neither of you likes it, that's fine with me :-)
>>>
>>>>
>>>> --
>>>> Cheers,
>>>>
>>>> David / dhildenb
>>>>
> 
> Thanks
> Barry

Re: [PATCH] mm: huge_memory: don't start_stop_khugepaged for non-PMD THP

[Barry Song]

On Fri, Jul 26, 2024 at 9:43 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>
> On 26/07/2024 09:28, Barry Song wrote:
> > From: Barry Song <v-songbaohua@oppo.com>
> >
> > khugepaged will be automatically started when PMD-sized THP is enabled
> > (either of the per-size anon control or the top-level control are set
> > to "always" or "madvise"), and it'll be automatically shutdown when
> > PMD-sized THP is disabled (when both the per-size anon control and the
> > top-level control are "never").
> >
> > It seems unnecessary to call start_stop_khugepaged() for non-PMD THP,
> > as it would only waste CPU time.
> >
> > Cc: Lance Yang <ioworker0@gmail.com>
> > Cc: Ryan Roberts <ryan.roberts@arm.com>
> > Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> > Cc: David Hildenbrand <david@redhat.com>
> > Cc: Yang Shi <shy828301@gmail.com>
> > Cc: Zi Yan <ziy@nvidia.com>
> > Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> > ---
> >  mm/huge_memory.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 41460847988c..bd365e35acf7 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -514,7 +514,7 @@ static ssize_t thpsize_enabled_store(struct kobject *kobj,
> >       } else
> >               ret = -EINVAL;
> >
> > -     if (ret > 0) {
> > +     if (ret > 0 && order == HPAGE_PMD_ORDER) {
> >               int err;
> >
> >               err = start_stop_khugepaged();
>
> Personally I see this as a bit of a layering violation; its
> start_stop_khugepaged() that should decide the policy for when to start and stop
> the daemon. thpsize_enabled_store() should just be calling
> start_stop_khugepaged() to notify that something potentially pertinent to the a
> policy decision has changed.
>
> And I don't think this should be a hot path; I'd expect the global thp size
> configuration to be set at boot and remain pretty static. Do you have evidence
> to the contrary?

Absolutely no, I was reading and patching the doc
"Documentation/admin-guide/mm/transhuge.rst"
and was also copying-paste your "enabled" code to "swapin_enabled" with dropping
start_stop_khugepaged() in that case. I believe I don't need it for
enabling/disabling
swapin as swapin is a subset of mTHP not a main entry.

just feel a bit inconsistency between the doc and the code at least for this
moment:
khugepaged will be automatically started when PMD-sized THP is enabled
(either of the per-size anon control or the top-level control are set
to "always" or "madvise"), and it'll be automatically shutdown when
PMD-sized THP is disabled (when both the per-size anon control and the
top-level control are "never").

which triggered me to make this change.

Thanks
Barry