[PATCH v2] mm/page_alloc: use batch page clearing in kernel_init_pages()

[PATCH v2] mm/page_alloc: use batch page clearing in kernel_init_pages()

[Hrushikesh Salunke]

When init_on_alloc is enabled, kernel_init_pages() clears every page
one at a time via clear_highpage_kasan_tagged(), which incurs per-page
kmap_local_page()/kunmap_local() overhead and prevents the architecture
clearing primitive from operating on contiguous ranges.

Introduce clear_highpages_kasan_tagged() in highmem.h, a batch
clearing helper that calls clear_pages() for the full contiguous range
on !HIGHMEM systems, bypassing the per-page kmap overhead and allowing
a single invocation of the arch clearing primitive across the entire
allocation. The HIGHMEM path falls back to per-page clearing since
those pages require kmap.

Use it in kernel_init_pages() to replace the per-page loop.

Allocating 8192 x 2MB HugeTLB pages (16GB) with init_on_alloc=1:

  Before: 0.445s
  After:  0.166s  (-62.7%, 2.68x faster)

Kernel time (sys) reduction per workload with init_on_alloc=1:

  Workload            Before       After       Change
  Graph500 64C128T    30m 41.8s    15m 14.8s   -50.3%
  Graph500 16C32T     15m 56.7s     9m 43.7s   -39.0%
  Pagerank 32T         1m 58.5s     1m 12.8s   -38.5%
  Pagerank 128T        2m 36.3s     1m 40.4s   -35.7%

Signed-off-by: Hrushikesh Salunke <hsalunke@amd.com>
---
base commit: f1541b40cd422d7e22273be9b7e9edfc9ea4f0d7

v1: https://lore.kernel.org/all/20260408092441.435133-1-hsalunke@amd.com/

Changes since v1:
- Dropped cond_resched() and PROCESS_PAGES_NON_PREEMPT_BATCH as
  kernel_init_pages() runs inside the page allocator and can be
  called from atomic context, making cond_resched() unsafe. The 
  original code never had a cond_resched() here, and the 
  performance gain comes from batching, not rescheduling.

- Moved the !HIGHMEM/HIGHMEM branching into a new
  clear_highpages_kasan_tagged() helper in highmem.h, per David's
  suggestion.

 include/linux/highmem.h | 12 ++++++++++++
 mm/page_alloc.c         |  5 +----
 2 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/include/linux/highmem.h b/include/linux/highmem.h
index af03db851a1d..ad0f42d06ce6 100644
--- a/include/linux/highmem.h
+++ b/include/linux/highmem.h
@@ -345,6 +345,18 @@ static inline void clear_highpage_kasan_tagged(struct page *page)
 	kunmap_local(kaddr);
 }
 
+static inline void clear_highpages_kasan_tagged(struct page *page, int numpages)
+{
+	if (!IS_ENABLED(CONFIG_HIGHMEM)) {
+		clear_pages(kasan_reset_tag(page_address(page)), numpages);
+	} else {
+		int i;
+
+		for (i = 0; i < numpages; i++)
+			clear_highpage_kasan_tagged(page + i);
+	}
+}
+
 #ifndef __HAVE_ARCH_TAG_CLEAR_HIGHPAGES
 
 /* Return false to let people know we did not initialize the pages */
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index b1c5430cad4e..1aaf7f839ff4 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1220,12 +1220,9 @@ static inline bool should_skip_kasan_poison(struct page *page)
 
 static void kernel_init_pages(struct page *page, int numpages)
 {
-	int i;
-
 	/* s390's use of memset() could override KASAN redzones. */
 	kasan_disable_current();
-	for (i = 0; i < numpages; i++)
-		clear_highpage_kasan_tagged(page + i);
+	clear_highpages_kasan_tagged(page, numpages);
 	kasan_enable_current();
 }
 
-- 
2.43.0

Re: [PATCH v2] mm/page_alloc: use batch page clearing in kernel_init_pages()

[Vlastimil Babka (SUSE)]

On 4/21/26 06:24, Hrushikesh Salunke wrote:
> When init_on_alloc is enabled, kernel_init_pages() clears every page
> one at a time via clear_highpage_kasan_tagged(), which incurs per-page
> kmap_local_page()/kunmap_local() overhead and prevents the architecture
> clearing primitive from operating on contiguous ranges.
> 
> Introduce clear_highpages_kasan_tagged() in highmem.h, a batch
> clearing helper that calls clear_pages() for the full contiguous range
> on !HIGHMEM systems, bypassing the per-page kmap overhead and allowing
> a single invocation of the arch clearing primitive across the entire
> allocation. The HIGHMEM path falls back to per-page clearing since
> those pages require kmap.
> 
> Use it in kernel_init_pages() to replace the per-page loop.
> 
> Allocating 8192 x 2MB HugeTLB pages (16GB) with init_on_alloc=1:
> 
>   Before: 0.445s
>   After:  0.166s  (-62.7%, 2.68x faster)
> 
> Kernel time (sys) reduction per workload with init_on_alloc=1:
> 
>   Workload            Before       After       Change
>   Graph500 64C128T    30m 41.8s    15m 14.8s   -50.3%
>   Graph500 16C32T     15m 56.7s     9m 43.7s   -39.0%
>   Pagerank 32T         1m 58.5s     1m 12.8s   -38.5%
>   Pagerank 128T        2m 36.3s     1m 40.4s   -35.7%
> 
> Signed-off-by: Hrushikesh Salunke <hsalunke@amd.com>
> ---
> base commit: f1541b40cd422d7e22273be9b7e9edfc9ea4f0d7
> 
> v1: https://lore.kernel.org/all/20260408092441.435133-1-hsalunke@amd.com/
> 
> Changes since v1:
> - Dropped cond_resched() and PROCESS_PAGES_NON_PREEMPT_BATCH as
>   kernel_init_pages() runs inside the page allocator and can be
>   called from atomic context, making cond_resched() unsafe. The 
>   original code never had a cond_resched() here, and the 
>   performance gain comes from batching, not rescheduling.
> 
> - Moved the !HIGHMEM/HIGHMEM branching into a new
>   clear_highpages_kasan_tagged() helper in highmem.h, per David's
>   suggestion.
> 
>  include/linux/highmem.h | 12 ++++++++++++
>  mm/page_alloc.c         |  5 +----
>  2 files changed, 13 insertions(+), 4 deletions(-)

Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

> 
> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> index af03db851a1d..ad0f42d06ce6 100644
> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -345,6 +345,18 @@ static inline void clear_highpage_kasan_tagged(struct page *page)
>  	kunmap_local(kaddr);
>  }
>  
> +static inline void clear_highpages_kasan_tagged(struct page *page, int numpages)
> +{
> +	if (!IS_ENABLED(CONFIG_HIGHMEM)) {
> +		clear_pages(kasan_reset_tag(page_address(page)), numpages);
> +	} else {
> +		int i;
> +
> +		for (i = 0; i < numpages; i++)
> +			clear_highpage_kasan_tagged(page + i);
> +	}
> +}
> +
>  #ifndef __HAVE_ARCH_TAG_CLEAR_HIGHPAGES
>  
>  /* Return false to let people know we did not initialize the pages */
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index b1c5430cad4e..1aaf7f839ff4 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1220,12 +1220,9 @@ static inline bool should_skip_kasan_poison(struct page *page)
>  
>  static void kernel_init_pages(struct page *page, int numpages)
>  {
> -	int i;
> -
>  	/* s390's use of memset() could override KASAN redzones. */
>  	kasan_disable_current();
> -	for (i = 0; i < numpages; i++)
> -		clear_highpage_kasan_tagged(page + i);
> +	clear_highpages_kasan_tagged(page, numpages);
>  	kasan_enable_current();
>  }
>  

Re: [PATCH v2] mm/page_alloc: use batch page clearing in kernel_init_pages()

[Zi Yan]

On 21 Apr 2026, at 0:24, Hrushikesh Salunke wrote:

> When init_on_alloc is enabled, kernel_init_pages() clears every page
> one at a time via clear_highpage_kasan_tagged(), which incurs per-page
> kmap_local_page()/kunmap_local() overhead and prevents the architecture
> clearing primitive from operating on contiguous ranges.
>
> Introduce clear_highpages_kasan_tagged() in highmem.h, a batch
> clearing helper that calls clear_pages() for the full contiguous range
> on !HIGHMEM systems, bypassing the per-page kmap overhead and allowing
> a single invocation of the arch clearing primitive across the entire
> allocation. The HIGHMEM path falls back to per-page clearing since
> those pages require kmap.
>
> Use it in kernel_init_pages() to replace the per-page loop.
>
> Allocating 8192 x 2MB HugeTLB pages (16GB) with init_on_alloc=1:
>
>   Before: 0.445s
>   After:  0.166s  (-62.7%, 2.68x faster)
>
> Kernel time (sys) reduction per workload with init_on_alloc=1:
>
>   Workload            Before       After       Change
>   Graph500 64C128T    30m 41.8s    15m 14.8s   -50.3%
>   Graph500 16C32T     15m 56.7s     9m 43.7s   -39.0%
>   Pagerank 32T         1m 58.5s     1m 12.8s   -38.5%
>   Pagerank 128T        2m 36.3s     1m 40.4s   -35.7%
>
> Signed-off-by: Hrushikesh Salunke <hsalunke@amd.com>
> ---
> base commit: f1541b40cd422d7e22273be9b7e9edfc9ea4f0d7
>
> v1: https://lore.kernel.org/all/20260408092441.435133-1-hsalunke@amd.com/
>
> Changes since v1:
> - Dropped cond_resched() and PROCESS_PAGES_NON_PREEMPT_BATCH as
>   kernel_init_pages() runs inside the page allocator and can be
>   called from atomic context, making cond_resched() unsafe. The
>   original code never had a cond_resched() here, and the
>   performance gain comes from batching, not rescheduling.
>
> - Moved the !HIGHMEM/HIGHMEM branching into a new
>   clear_highpages_kasan_tagged() helper in highmem.h, per David's
>   suggestion.
>
>  include/linux/highmem.h | 12 ++++++++++++
>  mm/page_alloc.c         |  5 +----
>  2 files changed, 13 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> index af03db851a1d..ad0f42d06ce6 100644
> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -345,6 +345,18 @@ static inline void clear_highpage_kasan_tagged(struct page *page)
>  	kunmap_local(kaddr);
>  }
>
> +static inline void clear_highpages_kasan_tagged(struct page *page, int numpages)
> +{
> +	if (!IS_ENABLED(CONFIG_HIGHMEM)) {
> +		clear_pages(kasan_reset_tag(page_address(page)), numpages);

kasan_reset_tag() here removes the tag from page address, so that
clear_pages() can use the right kaddr. I thought each page needs
a kasan_reset_tag(). No need to respond here, as I am reading
the code and trying to understand how it works.

Acked-by: Zi Yan <ziy@nvidia.com>


> +	} else {
> +		int i;
> +
> +		for (i = 0; i < numpages; i++)
> +			clear_highpage_kasan_tagged(page + i);
> +	}
> +}
> +
>  #ifndef __HAVE_ARCH_TAG_CLEAR_HIGHPAGES
>
>  /* Return false to let people know we did not initialize the pages */
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index b1c5430cad4e..1aaf7f839ff4 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1220,12 +1220,9 @@ static inline bool should_skip_kasan_poison(struct page *page)
>
>  static void kernel_init_pages(struct page *page, int numpages)
>  {
> -	int i;
> -
>  	/* s390's use of memset() could override KASAN redzones. */
>  	kasan_disable_current();
> -	for (i = 0; i < numpages; i++)
> -		clear_highpage_kasan_tagged(page + i);
> +	clear_highpages_kasan_tagged(page, numpages);
>  	kasan_enable_current();
>  }
>
> -- 
> 2.43.0


Best Regards,
Yan, Zi

Re: [PATCH v2] mm/page_alloc: use batch page clearing in kernel_init_pages()

[David Hildenbrand (Arm)]

On 4/21/26 15:44, Zi Yan wrote:
> On 21 Apr 2026, at 0:24, Hrushikesh Salunke wrote:
> 
>> When init_on_alloc is enabled, kernel_init_pages() clears every page
>> one at a time via clear_highpage_kasan_tagged(), which incurs per-page
>> kmap_local_page()/kunmap_local() overhead and prevents the architecture
>> clearing primitive from operating on contiguous ranges.
>>
>> Introduce clear_highpages_kasan_tagged() in highmem.h, a batch
>> clearing helper that calls clear_pages() for the full contiguous range
>> on !HIGHMEM systems, bypassing the per-page kmap overhead and allowing
>> a single invocation of the arch clearing primitive across the entire
>> allocation. The HIGHMEM path falls back to per-page clearing since
>> those pages require kmap.
>>
>> Use it in kernel_init_pages() to replace the per-page loop.
>>
>> Allocating 8192 x 2MB HugeTLB pages (16GB) with init_on_alloc=1:
>>
>>   Before: 0.445s
>>   After:  0.166s  (-62.7%, 2.68x faster)
>>
>> Kernel time (sys) reduction per workload with init_on_alloc=1:
>>
>>   Workload            Before       After       Change
>>   Graph500 64C128T    30m 41.8s    15m 14.8s   -50.3%
>>   Graph500 16C32T     15m 56.7s     9m 43.7s   -39.0%
>>   Pagerank 32T         1m 58.5s     1m 12.8s   -38.5%
>>   Pagerank 128T        2m 36.3s     1m 40.4s   -35.7%
>>
>> Signed-off-by: Hrushikesh Salunke <hsalunke@amd.com>
>> ---
>> base commit: f1541b40cd422d7e22273be9b7e9edfc9ea4f0d7
>>
>> v1: https://lore.kernel.org/all/20260408092441.435133-1-hsalunke@amd.com/
>>
>> Changes since v1:
>> - Dropped cond_resched() and PROCESS_PAGES_NON_PREEMPT_BATCH as
>>   kernel_init_pages() runs inside the page allocator and can be
>>   called from atomic context, making cond_resched() unsafe. The
>>   original code never had a cond_resched() here, and the
>>   performance gain comes from batching, not rescheduling.
>>
>> - Moved the !HIGHMEM/HIGHMEM branching into a new
>>   clear_highpages_kasan_tagged() helper in highmem.h, per David's
>>   suggestion.
>>
>>  include/linux/highmem.h | 12 ++++++++++++
>>  mm/page_alloc.c         |  5 +----
>>  2 files changed, 13 insertions(+), 4 deletions(-)
>>
>> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
>> index af03db851a1d..ad0f42d06ce6 100644
>> --- a/include/linux/highmem.h
>> +++ b/include/linux/highmem.h
>> @@ -345,6 +345,18 @@ static inline void clear_highpage_kasan_tagged(struct page *page)
>>  	kunmap_local(kaddr);
>>  }
>>
>> +static inline void clear_highpages_kasan_tagged(struct page *page, int numpages)
>> +{
>> +	if (!IS_ENABLED(CONFIG_HIGHMEM)) {
>> +		clear_pages(kasan_reset_tag(page_address(page)), numpages);
> 
> kasan_reset_tag() here removes the tag from page address, so that
> clear_pages() can use the right kaddr. I thought each page needs
> a kasan_reset_tag(). No need to respond here, as I am reading
> the code and trying to understand how it works.

It's all confusing. But we really just turn the pointer into an untagged
pointer here, once.

So I think this is ok.

I do wonder, though, whether we want to move the
kasan_disable_current/kasan_enable_current into the
clear_highpages_kasan_tagged().

-- 
Cheers,

David

Re: [PATCH v2] mm/page_alloc: use batch page clearing in kernel_init_pages()

[Zi Yan]

On 21 Apr 2026, at 9:57, David Hildenbrand (Arm) wrote:

> On 4/21/26 15:44, Zi Yan wrote:
>> On 21 Apr 2026, at 0:24, Hrushikesh Salunke wrote:
>>
>>> When init_on_alloc is enabled, kernel_init_pages() clears every page
>>> one at a time via clear_highpage_kasan_tagged(), which incurs per-page
>>> kmap_local_page()/kunmap_local() overhead and prevents the architecture
>>> clearing primitive from operating on contiguous ranges.
>>>
>>> Introduce clear_highpages_kasan_tagged() in highmem.h, a batch
>>> clearing helper that calls clear_pages() for the full contiguous range
>>> on !HIGHMEM systems, bypassing the per-page kmap overhead and allowing
>>> a single invocation of the arch clearing primitive across the entire
>>> allocation. The HIGHMEM path falls back to per-page clearing since
>>> those pages require kmap.
>>>
>>> Use it in kernel_init_pages() to replace the per-page loop.
>>>
>>> Allocating 8192 x 2MB HugeTLB pages (16GB) with init_on_alloc=1:
>>>
>>>   Before: 0.445s
>>>   After:  0.166s  (-62.7%, 2.68x faster)
>>>
>>> Kernel time (sys) reduction per workload with init_on_alloc=1:
>>>
>>>   Workload            Before       After       Change
>>>   Graph500 64C128T    30m 41.8s    15m 14.8s   -50.3%
>>>   Graph500 16C32T     15m 56.7s     9m 43.7s   -39.0%
>>>   Pagerank 32T         1m 58.5s     1m 12.8s   -38.5%
>>>   Pagerank 128T        2m 36.3s     1m 40.4s   -35.7%
>>>
>>> Signed-off-by: Hrushikesh Salunke <hsalunke@amd.com>
>>> ---
>>> base commit: f1541b40cd422d7e22273be9b7e9edfc9ea4f0d7
>>>
>>> v1: https://lore.kernel.org/all/20260408092441.435133-1-hsalunke@amd.com/
>>>
>>> Changes since v1:
>>> - Dropped cond_resched() and PROCESS_PAGES_NON_PREEMPT_BATCH as
>>>   kernel_init_pages() runs inside the page allocator and can be
>>>   called from atomic context, making cond_resched() unsafe. The
>>>   original code never had a cond_resched() here, and the
>>>   performance gain comes from batching, not rescheduling.
>>>
>>> - Moved the !HIGHMEM/HIGHMEM branching into a new
>>>   clear_highpages_kasan_tagged() helper in highmem.h, per David's
>>>   suggestion.
>>>
>>>  include/linux/highmem.h | 12 ++++++++++++
>>>  mm/page_alloc.c         |  5 +----
>>>  2 files changed, 13 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
>>> index af03db851a1d..ad0f42d06ce6 100644
>>> --- a/include/linux/highmem.h
>>> +++ b/include/linux/highmem.h
>>> @@ -345,6 +345,18 @@ static inline void clear_highpage_kasan_tagged(struct page *page)
>>>  	kunmap_local(kaddr);
>>>  }
>>>
>>> +static inline void clear_highpages_kasan_tagged(struct page *page, int numpages)
>>> +{
>>> +	if (!IS_ENABLED(CONFIG_HIGHMEM)) {
>>> +		clear_pages(kasan_reset_tag(page_address(page)), numpages);
>>
>> kasan_reset_tag() here removes the tag from page address, so that
>> clear_pages() can use the right kaddr. I thought each page needs
>> a kasan_reset_tag(). No need to respond here, as I am reading
>> the code and trying to understand how it works.
>
> It's all confusing. But we really just turn the pointer into an untagged
> pointer here, once.

Yes, I realized that after reading kasan_reset_tag() implementation.

>
> So I think this is ok.
>
> I do wonder, though, whether we want to move the
> kasan_disable_current/kasan_enable_current into the
> clear_highpages_kasan_tagged().

This sounds reasonable to me. And also replace kernel_init_pages()
with clear_highpages_kasan_tagged(), since kernel_init_pages()
will be a wrapper then.

Best Regards,
Yan, Zi

Re: [PATCH v2] mm/page_alloc: use batch page clearing in kernel_init_pages()

[David Hildenbrand (Arm)]

On 4/21/26 16:03, Zi Yan wrote:
> On 21 Apr 2026, at 9:57, David Hildenbrand (Arm) wrote:
> 
>> On 4/21/26 15:44, Zi Yan wrote:
>>>
>>>
>>> kasan_reset_tag() here removes the tag from page address, so that
>>> clear_pages() can use the right kaddr. I thought each page needs
>>> a kasan_reset_tag(). No need to respond here, as I am reading
>>> the code and trying to understand how it works.
>>
>> It's all confusing. But we really just turn the pointer into an untagged
>> pointer here, once.
> 
> Yes, I realized that after reading kasan_reset_tag() implementation.
> 
>>
>> So I think this is ok.
>>
>> I do wonder, though, whether we want to move the
>> kasan_disable_current/kasan_enable_current into the
>> clear_highpages_kasan_tagged().
> 
> This sounds reasonable to me. And also replace kernel_init_pages()
> with clear_highpages_kasan_tagged(), since kernel_init_pages()
> will be a wrapper then.

Agreed.

-- 
Cheers,

David