The khugepaged_scan_mm_slot() uses a 'progress' counter to limit the
amount of work performed and consists of three components:
1. Transitioning to a new mm (+1).
2. Skipping an unsuitable VMA (+1).
3. Scanning a PMD-sized range (+HPAGE_PMD_NR).
Consider a 1MB VMA sitting between two 2MB alignment boundaries:
vma1 vma2 vma3
+----------+------+----------+
|2M |1M |2M |
+----------+------+----------+
^ ^
start end
^
hstart,hend
In this case, for vma2:
hstart = round_up(start, HPAGE_PMD_SIZE) -> Next 2MB alignment
hend = round_down(end, HPAGE_PMD_SIZE) -> Prev 2MB alignment
Currently, since `hend <= hstart`, VMAs that are too small or unaligned
to contain a hugepage are skipped without incrementing 'progress'.
A process containing a large number of such small VMAs will unfairly
consume more CPU cycles before yielding compared to a process with
fewer, larger, or aligned VMAs.
Fix this by incrementing progress when the `hend <= hstart` condition
is met.
Additionally, change 'progress' type to `unsigned int` to match both
the 'pages' type and the function return value.
Suggested-by: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Shivank Garg <shivankg@amd.com>
---
mm/khugepaged.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 107146f012b1..0b549c3250f9 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2403,7 +2403,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
struct mm_slot *slot;
struct mm_struct *mm;
struct vm_area_struct *vma;
- int progress = 0;
+ unsigned int progress = 0;
VM_BUG_ON(!pages);
lockdep_assert_held(&khugepaged_mm_lock);
@@ -2447,7 +2447,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
}
hstart = round_up(vma->vm_start, HPAGE_PMD_SIZE);
hend = round_down(vma->vm_end, HPAGE_PMD_SIZE);
- if (khugepaged_scan.address > hend) {
+ if (khugepaged_scan.address > hend || hend <= hstart) {
progress++;
continue;
}
--
2.43.0
On 2025/12/24 19:13, Shivank Garg wrote:
> The khugepaged_scan_mm_slot() uses a 'progress' counter to limit the
> amount of work performed and consists of three components:
> 1. Transitioning to a new mm (+1).
> 2. Skipping an unsuitable VMA (+1).
> 3. Scanning a PMD-sized range (+HPAGE_PMD_NR).
>
> Consider a 1MB VMA sitting between two 2MB alignment boundaries:
>
> vma1 vma2 vma3
> +----------+------+----------+
> |2M |1M |2M |
> +----------+------+----------+
> ^ ^
> start end
> ^
> hstart,hend
>
> In this case, for vma2:
> hstart = round_up(start, HPAGE_PMD_SIZE) -> Next 2MB alignment
> hend = round_down(end, HPAGE_PMD_SIZE) -> Prev 2MB alignment
>
> Currently, since `hend <= hstart`, VMAs that are too small or unaligned
> to contain a hugepage are skipped without incrementing 'progress'.
> A process containing a large number of such small VMAs will unfairly
> consume more CPU cycles before yielding compared to a process with
> fewer, larger, or aligned VMAs.
>
> Fix this by incrementing progress when the `hend <= hstart` condition
> is met.
>
> Additionally, change 'progress' type to `unsigned int` to match both
> the 'pages' type and the function return value.
>
> Suggested-by: Wei Yang <richard.weiyang@gmail.com>
> Signed-off-by: Shivank Garg <shivankg@amd.com>
> ---
> mm/khugepaged.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 107146f012b1..0b549c3250f9 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -2403,7 +2403,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
> struct mm_slot *slot;
> struct mm_struct *mm;
> struct vm_area_struct *vma;
> - int progress = 0;
> + unsigned int progress = 0;
>
> VM_BUG_ON(!pages);
> lockdep_assert_held(&khugepaged_mm_lock);
> @@ -2447,7 +2447,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
> }
> hstart = round_up(vma->vm_start, HPAGE_PMD_SIZE);
> hend = round_down(vma->vm_end, HPAGE_PMD_SIZE);
> - if (khugepaged_scan.address > hend) {
Maybe add a short comment explaining why we increment progress for small
VMAs ;)
Something like this:
/* Count small VMAs that can't hold a hugepage towards scan limit */
> + if (khugepaged_scan.address > hend || hend <= hstart) {
> progress++;
> continue;
> }
Otherwise, looks good to me.
Reviewed-by: Lance Yang <lance.yang@linux.dev>
On Wed, Dec 24, 2025 at 07:51:36PM +0800, Lance Yang wrote:
>
>
>On 2025/12/24 19:13, Shivank Garg wrote:
>> The khugepaged_scan_mm_slot() uses a 'progress' counter to limit the
>> amount of work performed and consists of three components:
>> 1. Transitioning to a new mm (+1).
Hmm... maybe not only a new mm, but also we start another scan from last mm.
Since default khugepaged_pages_to_scan is 8 PMD, it looks very possible.
>> 2. Skipping an unsuitable VMA (+1).
>> 3. Scanning a PMD-sized range (+HPAGE_PMD_NR).
>>
>> Consider a 1MB VMA sitting between two 2MB alignment boundaries:
>>
>> vma1 vma2 vma3
>> +----------+------+----------+
>> |2M |1M |2M |
>> +----------+------+----------+
>> ^ ^
>> start end
>> ^
>> hstart,hend
>>
>> In this case, for vma2:
>> hstart = round_up(start, HPAGE_PMD_SIZE) -> Next 2MB alignment
>> hend = round_down(end, HPAGE_PMD_SIZE) -> Prev 2MB alignment
>>
>> Currently, since `hend <= hstart`, VMAs that are too small or unaligned
>> to contain a hugepage are skipped without incrementing 'progress'.
>> A process containing a large number of such small VMAs will unfairly
>> consume more CPU cycles before yielding compared to a process with
>> fewer, larger, or aligned VMAs.
>>
>> Fix this by incrementing progress when the `hend <= hstart` condition
>> is met.
>>
>> Additionally, change 'progress' type to `unsigned int` to match both
>> the 'pages' type and the function return value.
>>
>> Suggested-by: Wei Yang <richard.weiyang@gmail.com>
>> Signed-off-by: Shivank Garg <shivankg@amd.com>
>> ---
>> mm/khugepaged.c | 4 ++--
>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 107146f012b1..0b549c3250f9 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -2403,7 +2403,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
>> struct mm_slot *slot;
>> struct mm_struct *mm;
>> struct vm_area_struct *vma;
>> - int progress = 0;
>> + unsigned int progress = 0;
>> VM_BUG_ON(!pages);
>> lockdep_assert_held(&khugepaged_mm_lock);
>> @@ -2447,7 +2447,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
>> }
>> hstart = round_up(vma->vm_start, HPAGE_PMD_SIZE);
>> hend = round_down(vma->vm_end, HPAGE_PMD_SIZE);
>> - if (khugepaged_scan.address > hend) {
>
>Maybe add a short comment explaining why we increment progress for small VMAs
>;)
>
>Something like this:
>
> /* Count small VMAs that can't hold a hugepage towards scan limit */
>> + if (khugepaged_scan.address > hend || hend <= hstart) {
>> progress++;
>> continue;
>> }
>
>Otherwise, looks good to me.
>
>Reviewed-by: Lance Yang <lance.yang@linux.dev>
>
The code change LGTM.
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
--
Wei Yang
Help you, Help me
On 12/24/2025 8:19 PM, Wei Yang wrote:
> On Wed, Dec 24, 2025 at 07:51:36PM +0800, Lance Yang wrote:
>>
>>
>> On 2025/12/24 19:13, Shivank Garg wrote:
>>> The khugepaged_scan_mm_slot() uses a 'progress' counter to limit the
>>> amount of work performed and consists of three components:
>>> 1. Transitioning to a new mm (+1).
>
> Hmm... maybe not only a new mm, but also we start another scan from last mm.
>
> Since default khugepaged_pages_to_scan is 8 PMD, it looks very possible.
>
It makes sense, will correct this.
>>> 2. Skipping an unsuitable VMA (+1).
>>> 3. Scanning a PMD-sized range (+HPAGE_PMD_NR).
>>>
>>> Consider a 1MB VMA sitting between two 2MB alignment boundaries:
>>>
>>> vma1 vma2 vma3
>>> +----------+------+----------+
>>> |2M |1M |2M |
>>> +----------+------+----------+
>>> ^ ^
>>> start end
>>> ^
>>> hstart,hend
>>>
>>> In this case, for vma2:
>>> hstart = round_up(start, HPAGE_PMD_SIZE) -> Next 2MB alignment
>>> hend = round_down(end, HPAGE_PMD_SIZE) -> Prev 2MB alignment
>>>
>>> Currently, since `hend <= hstart`, VMAs that are too small or unaligned
>>> to contain a hugepage are skipped without incrementing 'progress'.
>>> A process containing a large number of such small VMAs will unfairly
>>> consume more CPU cycles before yielding compared to a process with
>>> fewer, larger, or aligned VMAs.
>>>
>>> Fix this by incrementing progress when the `hend <= hstart` condition
>>> is met.
>>>
>>> Additionally, change 'progress' type to `unsigned int` to match both
>>> the 'pages' type and the function return value.
>>>
>>> Suggested-by: Wei Yang <richard.weiyang@gmail.com>
>>> Signed-off-by: Shivank Garg <shivankg@amd.com>
>>> ---
>>> mm/khugepaged.c | 4 ++--
>>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>> index 107146f012b1..0b549c3250f9 100644
>>> --- a/mm/khugepaged.c
>>> +++ b/mm/khugepaged.c
>>> @@ -2403,7 +2403,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
>>> struct mm_slot *slot;
>>> struct mm_struct *mm;
>>> struct vm_area_struct *vma;
>>> - int progress = 0;
>>> + unsigned int progress = 0;
>>> VM_BUG_ON(!pages);
>>> lockdep_assert_held(&khugepaged_mm_lock);
>>> @@ -2447,7 +2447,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
>>> }
>>> hstart = round_up(vma->vm_start, HPAGE_PMD_SIZE);
>>> hend = round_down(vma->vm_end, HPAGE_PMD_SIZE);
>>> - if (khugepaged_scan.address > hend) {
>>
>> Maybe add a short comment explaining why we increment progress for small VMAs
>> ;)
>>
>> Something like this:
>>
>> /* Count small VMAs that can't hold a hugepage towards scan limit */
I'll add explanation.
>>> + if (khugepaged_scan.address > hend || hend <= hstart) {
>>> progress++;
>>> continue;
>>> }
>>
>> Otherwise, looks good to me.
>>
>> Reviewed-by: Lance Yang <lance.yang@linux.dev>
>>
>
> The code change LGTM.
>
> Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
>
Thanks Lance and Wei. I have made suggested changes.
From d464604c09cef70f0f2aa0f9607a977b4bcd7081 Mon Sep 17 00:00:00 2001
From: Shivank Garg <shivankg@amd.com>
Date: Wed, 17 Dec 2025 19:43:26 +0000
Subject: [PATCH V2 2/5] mm/khugepaged: count small VMAs towards scan limit
The khugepaged_scan_mm_slot() uses a 'progress' counter to limit the
amount of work performed and consists of three components:
1. Starting/resuming scan of mm slot (+1).
2. Skipping an unsuitable VMA (+1).
3. Scanning a PMD-sized range (+HPAGE_PMD_NR).
Consider a 1MB VMA sitting between two 2MB alignment boundaries:
vma1 vma2 vma3
+----------+------+----------+
|2M |1M |2M |
+----------+------+----------+
^ ^
start end
^
hstart,hend
In this case, for vma2:
hstart = round_up(start, HPAGE_PMD_SIZE) -> Next 2MB alignment
hend = round_down(end, HPAGE_PMD_SIZE) -> Prev 2MB alignment
Currently, since `hend <= hstart`, VMAs that are too small or unaligned
to contain a hugepage are skipped without incrementing 'progress'.
A process containing a large number of such small VMAs will unfairly
consume more CPU cycles before yielding compared to a process with
fewer, larger, or aligned VMAs.
Fix this by incrementing progress when the `hend <= hstart` condition
is met.
Additionally, change 'progress' to `unsigned int`. This matches both
the 'pages' type and the function return value.
Suggested-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Signed-off-by: Shivank Garg <shivankg@amd.com>
---
mm/khugepaged.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 107146f012b1..155281c49169 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2403,7 +2403,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
struct mm_slot *slot;
struct mm_struct *mm;
struct vm_area_struct *vma;
- int progress = 0;
+ unsigned int progress = 0;
VM_BUG_ON(!pages);
lockdep_assert_held(&khugepaged_mm_lock);
@@ -2447,7 +2447,8 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
}
hstart = round_up(vma->vm_start, HPAGE_PMD_SIZE);
hend = round_down(vma->vm_end, HPAGE_PMD_SIZE);
- if (khugepaged_scan.address > hend) {
+ if (khugepaged_scan.address > hend || hend <= hstart) {
+ /* VMA already scanned or too small/unaligned for hugepage. */
progress++;
continue;
}
--
2.43.0
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