[PATCH v3 0/2] mm: introduce per-order mTHP split counters

[PATCH v3 0/2] mm: introduce per-order mTHP split counters

[Lance Yang]

Hi all,

At present, the split counters in THP statistics no longer include
PTE-mapped mTHP. Therefore, we want to introduce per-order mTHP split
counters to monitor the frequency of mTHP splits. This will assist
developers in better analyzing and optimizing system performance.

/sys/kernel/mm/transparent_hugepage/hugepages-<size>/stats
        split
        split_failed
        split_deferred

---

Changes since v2 [2]
====================
 - mm: add per-order mTHP split counters
   - Pick AB from Barry - thanks!
   - Pick RB from Baolin - thanks!
   - Pick RB from Ryan - thanks!
   - Make things more readable (per Barry and Baolin)
 - mm: add docs for per-order mTHP split counters
   - Pick RB from Barry - thanks!
   - Improve the doc as suggested by Ryan
   - Remove the outdated note (per Ryan)

Changes since v1 [1]
====================
 - mm: add per-order mTHP split counters
   - Update the changelog
   - Drop '_page' from mTHP split counter names (per David and Ryan)
   - Store the order of the folio in a variable and reuse it later
     (per Bang)
 - mm: add docs for per-order mTHP split counters
   - Improve the doc as suggested by Ryan

[1] https://lore.kernel.org/linux-mm/20240424135148.30422-1-ioworker0@gmail.com
[2] https://lore.kernel.org/linux-mm/20240628130750.73097-1-ioworker0@gmail.com

Lance Yang (2):
  mm: add per-order mTHP split counters
  mm: add docs for per-order mTHP split counters

 Documentation/admin-guide/mm/transhuge.rst | 20 ++++++++++++++++----
 include/linux/huge_mm.h                    |  3 +++
 mm/huge_memory.c                           | 12 ++++++++++--
 3 files changed, 29 insertions(+), 6 deletions(-)


base-commit: ec34ecf3924f09a694e5c7d8d7e785b82f67f9f0
-- 
2.45.2

[PATCH v3 1/2] mm: add per-order mTHP split counters

[Lance Yang]

Currently, the split counters in THP statistics no longer include
PTE-mapped mTHP. Therefore, we propose introducing per-order mTHP split
counters to monitor the frequency of mTHP splits. This will help developers
better analyze and optimize system performance.

/sys/kernel/mm/transparent_hugepage/hugepages-<size>/stats
        split
        split_failed
        split_deferred

Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: Barry Song <baohua@kernel.org>
Signed-off-by: Mingzhe Yang <mingzhe.yang@ly.com>
Signed-off-by: Lance Yang <ioworker0@gmail.com>
---
 include/linux/huge_mm.h |  3 +++
 mm/huge_memory.c        | 12 ++++++++++--
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 212cca384d7e..cee3c5da8f0e 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -284,6 +284,9 @@ enum mthp_stat_item {
 	MTHP_STAT_FILE_ALLOC,
 	MTHP_STAT_FILE_FALLBACK,
 	MTHP_STAT_FILE_FALLBACK_CHARGE,
+	MTHP_STAT_SPLIT,
+	MTHP_STAT_SPLIT_FAILED,
+	MTHP_STAT_SPLIT_DEFERRED,
 	__MTHP_STAT_COUNT
 };
 
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 954c63575917..5cbd838e96e6 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -559,6 +559,9 @@ DEFINE_MTHP_STAT_ATTR(swpout_fallback, MTHP_STAT_SWPOUT_FALLBACK);
 DEFINE_MTHP_STAT_ATTR(file_alloc, MTHP_STAT_FILE_ALLOC);
 DEFINE_MTHP_STAT_ATTR(file_fallback, MTHP_STAT_FILE_FALLBACK);
 DEFINE_MTHP_STAT_ATTR(file_fallback_charge, MTHP_STAT_FILE_FALLBACK_CHARGE);
+DEFINE_MTHP_STAT_ATTR(split, MTHP_STAT_SPLIT);
+DEFINE_MTHP_STAT_ATTR(split_failed, MTHP_STAT_SPLIT_FAILED);
+DEFINE_MTHP_STAT_ATTR(split_deferred, MTHP_STAT_SPLIT_DEFERRED);
 
 static struct attribute *stats_attrs[] = {
 	&anon_fault_alloc_attr.attr,
@@ -569,6 +572,9 @@ static struct attribute *stats_attrs[] = {
 	&file_alloc_attr.attr,
 	&file_fallback_attr.attr,
 	&file_fallback_charge_attr.attr,
+	&split_attr.attr,
+	&split_failed_attr.attr,
+	&split_deferred_attr.attr,
 	NULL,
 };
 
@@ -3068,7 +3074,7 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
 	XA_STATE_ORDER(xas, &folio->mapping->i_pages, folio->index, new_order);
 	struct anon_vma *anon_vma = NULL;
 	struct address_space *mapping = NULL;
-	bool is_thp = folio_test_pmd_mappable(folio);
+	int order = folio_order(folio);
 	int extra_pins, ret;
 	pgoff_t end;
 	bool is_hzp;
@@ -3253,8 +3259,9 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
 		i_mmap_unlock_read(mapping);
 out:
 	xas_destroy(&xas);
-	if (is_thp)
+	if (order >= HPAGE_PMD_ORDER)
 		count_vm_event(!ret ? THP_SPLIT_PAGE : THP_SPLIT_PAGE_FAILED);
+	count_mthp_stat(order, !ret ? MTHP_STAT_SPLIT : MTHP_STAT_SPLIT_FAILED);
 	return ret;
 }
 
@@ -3307,6 +3314,7 @@ void deferred_split_folio(struct folio *folio)
 	if (list_empty(&folio->_deferred_list)) {
 		if (folio_test_pmd_mappable(folio))
 			count_vm_event(THP_DEFERRED_SPLIT_PAGE);
+		count_mthp_stat(folio_order(folio), MTHP_STAT_SPLIT_DEFERRED);
 		list_add_tail(&folio->_deferred_list, &ds_queue->split_queue);
 		ds_queue->split_queue_len++;
 #ifdef CONFIG_MEMCG
-- 
2.45.2

[PATCH v3 2/2] mm: add docs for per-order mTHP split counters

[Lance Yang]

This commit introduces documentation for mTHP split counters in
transhuge.rst.

Reviewed-by: Barry Song <baohua@kernel.org>
Signed-off-by: Mingzhe Yang <mingzhe.yang@ly.com>
Signed-off-by: Lance Yang <ioworker0@gmail.com>
---
 Documentation/admin-guide/mm/transhuge.rst | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index 1f72b00af5d3..0830aa173a8b 100644
--- a/Documentation/admin-guide/mm/transhuge.rst
+++ b/Documentation/admin-guide/mm/transhuge.rst
@@ -369,10 +369,6 @@ also applies to the regions registered in khugepaged.
 Monitoring usage
 ================
 
-.. note::
-   Currently the below counters only record events relating to
-   PMD-sized THP. Events relating to other THP sizes are not included.
-
 The number of PMD-sized anonymous transparent huge pages currently used by the
 system is available by reading the AnonHugePages field in ``/proc/meminfo``.
 To identify what applications are using PMD-sized anonymous transparent huge
@@ -514,6 +510,22 @@ file_fallback_charge
 	falls back to using small pages even though the allocation was
 	successful.
 
+split
+	is incremented every time a huge page is successfully split into
+	smaller orders. This can happen for a variety of reasons but a
+	common reason is that a huge page is old and is being reclaimed.
+	This action implies splitting any block mappings into PTEs.
+
+split_failed
+	is incremented if kernel fails to split huge
+	page. This can happen if the page was pinned by somebody.
+
+split_deferred
+	is incremented when a huge page is put onto split
+	queue. This happens when a huge page is partially unmapped and
+	splitting it would free up some memory. Pages on split queue are
+	going to be split under memory pressure.
+
 As the system ages, allocating huge pages may be expensive as the
 system uses memory compaction to copy data around memory to free a
 huge page for use. There are some counters in ``/proc/vmstat`` to help
-- 
2.45.2

Re: [PATCH v3 1/2] mm: add per-order mTHP split counters

[David Hildenbrand]

> @@ -3253,8 +3259,9 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>   		i_mmap_unlock_read(mapping);
>   out:
>   	xas_destroy(&xas);
> -	if (is_thp)
> +	if (order >= HPAGE_PMD_ORDER)

We likely should be using "== HPAGE_PMD_ORDER" here, to be safe for the 
future.

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb

Re: [PATCH v3 2/2] mm: add docs for per-order mTHP split counters

[David Hildenbrand]

On 04.07.24 03:29, Lance Yang wrote:
> This commit introduces documentation for mTHP split counters in
> transhuge.rst.
> 
> Reviewed-by: Barry Song <baohua@kernel.org>
> Signed-off-by: Mingzhe Yang <mingzhe.yang@ly.com>
> Signed-off-by: Lance Yang <ioworker0@gmail.com>
> ---
>   Documentation/admin-guide/mm/transhuge.rst | 20 ++++++++++++++++----
>   1 file changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index 1f72b00af5d3..0830aa173a8b 100644
> --- a/Documentation/admin-guide/mm/transhuge.rst
> +++ b/Documentation/admin-guide/mm/transhuge.rst
> @@ -369,10 +369,6 @@ also applies to the regions registered in khugepaged.
>   Monitoring usage
>   ================
>   
> -.. note::
> -   Currently the below counters only record events relating to
> -   PMD-sized THP. Events relating to other THP sizes are not included.
> -
>   The number of PMD-sized anonymous transparent huge pages currently used by the
>   system is available by reading the AnonHugePages field in ``/proc/meminfo``.
>   To identify what applications are using PMD-sized anonymous transparent huge
> @@ -514,6 +510,22 @@ file_fallback_charge
>   	falls back to using small pages even though the allocation was
>   	successful.
>   
> +split
> +	is incremented every time a huge page is successfully split into
> +	smaller orders. This can happen for a variety of reasons but a
> +	common reason is that a huge page is old and is being reclaimed.
> +	This action implies splitting any block mappings into PTEs.
> +
> +split_failed
> +	is incremented if kernel fails to split huge
> +	page. This can happen if the page was pinned by somebody.
> +
> +split_deferred
> +	is incremented when a huge page is put onto split
> +	queue. This happens when a huge page is partially unmapped and
> +	splitting it would free up some memory. Pages on split queue are
> +	going to be split under memory pressure.

".. if splitting is possible." ;)

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb

Re: [PATCH v3 2/2] mm: add docs for per-order mTHP split counters

[Ryan Roberts]

On 04/07/2024 02:29, Lance Yang wrote:
> This commit introduces documentation for mTHP split counters in
> transhuge.rst.
> 
> Reviewed-by: Barry Song <baohua@kernel.org>
> Signed-off-by: Mingzhe Yang <mingzhe.yang@ly.com>
> Signed-off-by: Lance Yang <ioworker0@gmail.com>
> ---
>  Documentation/admin-guide/mm/transhuge.rst | 20 ++++++++++++++++----
>  1 file changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index 1f72b00af5d3..0830aa173a8b 100644
> --- a/Documentation/admin-guide/mm/transhuge.rst
> +++ b/Documentation/admin-guide/mm/transhuge.rst
> @@ -369,10 +369,6 @@ also applies to the regions registered in khugepaged.
>  Monitoring usage
>  ================
>  
> -.. note::
> -   Currently the below counters only record events relating to
> -   PMD-sized THP. Events relating to other THP sizes are not included.
> -
>  The number of PMD-sized anonymous transparent huge pages currently used by the
>  system is available by reading the AnonHugePages field in ``/proc/meminfo``.
>  To identify what applications are using PMD-sized anonymous transparent huge
> @@ -514,6 +510,22 @@ file_fallback_charge
>  	falls back to using small pages even though the allocation was
>  	successful.
>  
> +split
> +	is incremented every time a huge page is successfully split into
> +	smaller orders. This can happen for a variety of reasons but a
> +	common reason is that a huge page is old and is being reclaimed.
> +	This action implies splitting any block mappings into PTEs.

nit: the block mappings will already be PTEs if starting with mTHP?

regardless:

Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>

> +
> +split_failed
> +	is incremented if kernel fails to split huge
> +	page. This can happen if the page was pinned by somebody.
> +
> +split_deferred
> +	is incremented when a huge page is put onto split
> +	queue. This happens when a huge page is partially unmapped and
> +	splitting it would free up some memory. Pages on split queue are
> +	going to be split under memory pressure.
> +
>  As the system ages, allocating huge pages may be expensive as the
>  system uses memory compaction to copy data around memory to free a
>  huge page for use. There are some counters in ``/proc/vmstat`` to help

Re: [PATCH v3 2/2] mm: add docs for per-order mTHP split counters

[David Hildenbrand]

On 05.07.24 11:16, Ryan Roberts wrote:
> On 04/07/2024 02:29, Lance Yang wrote:
>> This commit introduces documentation for mTHP split counters in
>> transhuge.rst.
>>
>> Reviewed-by: Barry Song <baohua@kernel.org>
>> Signed-off-by: Mingzhe Yang <mingzhe.yang@ly.com>
>> Signed-off-by: Lance Yang <ioworker0@gmail.com>
>> ---
>>   Documentation/admin-guide/mm/transhuge.rst | 20 ++++++++++++++++----
>>   1 file changed, 16 insertions(+), 4 deletions(-)
>>
>> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
>> index 1f72b00af5d3..0830aa173a8b 100644
>> --- a/Documentation/admin-guide/mm/transhuge.rst
>> +++ b/Documentation/admin-guide/mm/transhuge.rst
>> @@ -369,10 +369,6 @@ also applies to the regions registered in khugepaged.
>>   Monitoring usage
>>   ================
>>   
>> -.. note::
>> -   Currently the below counters only record events relating to
>> -   PMD-sized THP. Events relating to other THP sizes are not included.
>> -
>>   The number of PMD-sized anonymous transparent huge pages currently used by the
>>   system is available by reading the AnonHugePages field in ``/proc/meminfo``.
>>   To identify what applications are using PMD-sized anonymous transparent huge
>> @@ -514,6 +510,22 @@ file_fallback_charge
>>   	falls back to using small pages even though the allocation was
>>   	successful.
>>   
>> +split
>> +	is incremented every time a huge page is successfully split into
>> +	smaller orders. This can happen for a variety of reasons but a
>> +	common reason is that a huge page is old and is being reclaimed.
>> +	This action implies splitting any block mappings into PTEs.
> 
> nit: the block mappings will already be PTEs if starting with mTHP?


Was confused by that as well, so maybe just drop that detail here.
-- 
Cheers,

David / dhildenb

Re: [PATCH v3 1/2] mm: add per-order mTHP split counters

[Barry Song]

On Fri, Jul 5, 2024 at 9:08 PM David Hildenbrand <david@redhat.com> wrote:
>
> > @@ -3253,8 +3259,9 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
> >               i_mmap_unlock_read(mapping);
> >   out:
> >       xas_destroy(&xas);
> > -     if (is_thp)
> > +     if (order >= HPAGE_PMD_ORDER)
>
> We likely should be using "== HPAGE_PMD_ORDER" here, to be safe for the
> future.

I feel this might need to be separate since all other places are using
folio_test_pmd_mappable() ?

>
> Acked-by: David Hildenbrand <david@redhat.com>
>
> --
> Cheers,
>
> David / dhildenb
>

Re: [PATCH v3 1/2] mm: add per-order mTHP split counters

[David Hildenbrand]

On 05.07.24 12:12, Barry Song wrote:
> On Fri, Jul 5, 2024 at 9:08 PM David Hildenbrand <david@redhat.com> wrote:
>>
>>> @@ -3253,8 +3259,9 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>>>                i_mmap_unlock_read(mapping);
>>>    out:
>>>        xas_destroy(&xas);
>>> -     if (is_thp)
>>> +     if (order >= HPAGE_PMD_ORDER)
>>
>> We likely should be using "== HPAGE_PMD_ORDER" here, to be safe for the
>> future.
> 
> I feel this might need to be separate since all other places are using
> folio_test_pmd_mappable() ?

Likely, but as you are moving away from this ... this counter here does 
and will always only care about HPAGE_PMD_ORDER.

-- 
Cheers,

David / dhildenb

Re: [PATCH v3 1/2] mm: add per-order mTHP split counters

[Lance Yang]

Hi David and Barry,

Thanks a lot for paying attention!

On Fri, Jul 5, 2024 at 6:14 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 05.07.24 12:12, Barry Song wrote:
> > On Fri, Jul 5, 2024 at 9:08 PM David Hildenbrand <david@redhat.com> wrote:
> >>
> >>> @@ -3253,8 +3259,9 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
> >>>                i_mmap_unlock_read(mapping);
> >>>    out:
> >>>        xas_destroy(&xas);
> >>> -     if (is_thp)
> >>> +     if (order >= HPAGE_PMD_ORDER)
> >>
> >> We likely should be using "== HPAGE_PMD_ORDER" here, to be safe for the
> >> future.
> >
> > I feel this might need to be separate since all other places are using
> > folio_test_pmd_mappable() ?
>
> Likely, but as you are moving away from this ... this counter here does
> and will always only care about HPAGE_PMD_ORDER.

I appreciate the different opinions on whether we should use
">= HPAGE_PMD_ORDER" or "==" for this check.

In this context, let's leave it as is and stay consistent with
folio_test_pmd_mappable() by using ">= HPAGE_PMD_ORDER",
what do you think?

Thanks,
Lance

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

Re: [PATCH v3 1/2] mm: add per-order mTHP split counters

[David Hildenbrand]

On 05.07.24 12:48, Lance Yang wrote:
> Hi David and Barry,
> 
> Thanks a lot for paying attention!
> 
> On Fri, Jul 5, 2024 at 6:14 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 05.07.24 12:12, Barry Song wrote:
>>> On Fri, Jul 5, 2024 at 9:08 PM David Hildenbrand <david@redhat.com> wrote:
>>>>
>>>>> @@ -3253,8 +3259,9 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>>>>>                 i_mmap_unlock_read(mapping);
>>>>>     out:
>>>>>         xas_destroy(&xas);
>>>>> -     if (is_thp)
>>>>> +     if (order >= HPAGE_PMD_ORDER)
>>>>
>>>> We likely should be using "== HPAGE_PMD_ORDER" here, to be safe for the
>>>> future.
>>>
>>> I feel this might need to be separate since all other places are using
>>> folio_test_pmd_mappable() ?
>>
>> Likely, but as you are moving away from this ... this counter here does
>> and will always only care about HPAGE_PMD_ORDER.
> 
> I appreciate the different opinions on whether we should use
> ">= HPAGE_PMD_ORDER" or "==" for this check.
> 
> In this context, let's leave it as is and stay consistent with
> folio_test_pmd_mappable() by using ">= HPAGE_PMD_ORDER",
> what do you think?

I don't think it's a good idea to add more wrong code that is even 
harder to grep (folio_test_pmd_mappable would give you candidates that 
might need attention). But I don't care too much. Maybe someone here can 
volunteer to clean up these instances to make sure we check PMD-size and 
not PMD-mappable for these counters that are for PMD-sized folios only, 
even in the future with larger folios?

-- 
Cheers,

David / dhildenb

Re: [PATCH v3 2/2] mm: add docs for per-order mTHP split counters

[Lance Yang]

Hi David and Ryan,

Thanks for taking time to review!

Updated the doc. How about the following?

split
	is incremented every time a huge page is successfully split into
	smaller orders. This can happen for a variety of reasons but a
	common reason is that a huge page is old and is being reclaimed.

split_failed
	is incremented if kernel fails to split huge
	page. This can happen if the page was pinned by somebody.

split_deferred
	is incremented when a huge page is put onto split queue.
	This happens when a huge page is partially unmapped and splitting
	it would free up some memory. Pages on split queue are going to
	be split under memory pressure, if splitting is possible.

Thanks,
Lance

Re: [PATCH v3 1/2] mm: add per-order mTHP split counters

[Lance Yang]

On Fri, Jul 5, 2024 at 6:56 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 05.07.24 12:48, Lance Yang wrote:
> > Hi David and Barry,
> >
> > Thanks a lot for paying attention!
> >
> > On Fri, Jul 5, 2024 at 6:14 PM David Hildenbrand <david@redhat.com> wrote:
> >>
> >> On 05.07.24 12:12, Barry Song wrote:
> >>> On Fri, Jul 5, 2024 at 9:08 PM David Hildenbrand <david@redhat.com> wrote:
> >>>>
> >>>>> @@ -3253,8 +3259,9 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
> >>>>>                 i_mmap_unlock_read(mapping);
> >>>>>     out:
> >>>>>         xas_destroy(&xas);
> >>>>> -     if (is_thp)
> >>>>> +     if (order >= HPAGE_PMD_ORDER)
> >>>>
> >>>> We likely should be using "== HPAGE_PMD_ORDER" here, to be safe for the
> >>>> future.
> >>>
> >>> I feel this might need to be separate since all other places are using
> >>> folio_test_pmd_mappable() ?
> >>
> >> Likely, but as you are moving away from this ... this counter here does
> >> and will always only care about HPAGE_PMD_ORDER.
> >
> > I appreciate the different opinions on whether we should use
> > ">= HPAGE_PMD_ORDER" or "==" for this check.
> >
> > In this context, let's leave it as is and stay consistent with
> > folio_test_pmd_mappable() by using ">= HPAGE_PMD_ORDER",
> > what do you think?
>
> I don't think it's a good idea to add more wrong code that is even
> harder to grep (folio_test_pmd_mappable would give you candidates that
> might need attention). But I don't care too much. Maybe someone here can
> volunteer to clean up these instances to make sure we check PMD-size and
> not PMD-mappable for these counters that are for PMD-sized folios only,
> even in the future with larger folios?

Thanks for clarifying! Yes, agreed. We should ensure we check PMD-size,
not PMD-mappable here, especially as we consider large folios in the future.

So, let's use "== HPAGE_PMD_ORDER" here ;)

Thanks,
Lance

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

Re: [PATCH v3 1/2] mm: add per-order mTHP split counters

[Lance Yang]

Hi Andrew,

Could you please fold the following changes into this patch?

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 993f0dd2a1ed..f8b5cbd4dd71 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3266,7 +3266,7 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
 		i_mmap_unlock_read(mapping);
 out:
 	xas_destroy(&xas);
-	if (order >= HPAGE_PMD_ORDER)
+	if (order == HPAGE_PMD_ORDER)
 		count_vm_event(!ret ? THP_SPLIT_PAGE : THP_SPLIT_PAGE_FAILED);
 	count_mthp_stat(order, !ret ? MTHP_STAT_SPLIT : MTHP_STAT_SPLIT_FAILED);
 	return ret;

Thanks,
Lance

Re: [PATCH v3 2/2] mm: add docs for per-order mTHP split counters

[Lance Yang]

Hi Andrew,

Could you please fold the following changes into this patch?

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index 747c811ee8f1..fe237825b95c 100644
--- a/Documentation/admin-guide/mm/transhuge.rst
+++ b/Documentation/admin-guide/mm/transhuge.rst
@@ -513,17 +513,16 @@ split
 	is incremented every time a huge page is successfully split into
 	smaller orders. This can happen for a variety of reasons but a
 	common reason is that a huge page is old and is being reclaimed.
-	This action implies splitting any block mappings into PTEs.
 
 split_failed
 	is incremented if kernel fails to split huge
 	page. This can happen if the page was pinned by somebody.
 
 split_deferred
-	is incremented when a huge page is put onto split
-	queue. This happens when a huge page is partially unmapped and
-	splitting it would free up some memory. Pages on split queue are
-	going to be split under memory pressure.
+        is incremented when a huge page is put onto split queue.
+        This happens when a huge page is partially unmapped and splitting
+        it would free up some memory. Pages on split queue are going to
+        be split under memory pressure, if splitting is possible.
 
 As the system ages, allocating huge pages may be expensive as the
 system uses memory compaction to copy data around memory to free a
-- 

Thanks,
Lance