add static PMD zero page support

[PATCH v2 4/5] mm: add largest_zero_folio() routine

Posted by Pankaj Raghav (Samsung) 3 months ago

From: Pankaj Raghav <p.raghav@samsung.com>

Add largest_zero_folio() routine so that huge_zero_folio can be
used without the need to pass any mm struct. This will return ZERO_PAGE
folio if CONFIG_STATIC_PMD_ZERO_PAGE is disabled or if we failed to
allocate a PMD page from memblock.

This routine can also be called even if THP is disabled.

Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
---
 include/linux/mm.h | 28 ++++++++++++++++++++++++++--
 1 file changed, 26 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 428fe6d36b3c..d5543cf7b8e9 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -4018,17 +4018,41 @@ static inline bool vma_is_special_huge(const struct vm_area_struct *vma)
 
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE || CONFIG_HUGETLBFS */
 
+extern struct folio *huge_zero_folio;
+extern unsigned long huge_zero_pfn;
+
 #ifdef CONFIG_STATIC_PMD_ZERO_PAGE
 extern void __init static_pmd_zero_init(void);
+
+/*
+ * largest_zero_folio - Get the largest zero size folio available
+ *
+ * This function will return a PMD sized zero folio if CONFIG_STATIC_PMD_ZERO_PAGE
+ * is enabled. Otherwise, a ZERO_PAGE folio is returned.
+ *
+ * Deduce the size of the folio with folio_size instead of assuming the
+ * folio size.
+ */
+static inline struct folio *largest_zero_folio(void)
+{
+	if(!huge_zero_folio)
+		return page_folio(ZERO_PAGE(0));
+
+	return READ_ONCE(huge_zero_folio);
+}
+
 #else
 static inline void __init static_pmd_zero_init(void)
 {
 	return;
 }
+
+static inline struct folio *largest_zero_folio(void)
+{
+	return page_folio(ZERO_PAGE(0));
+}
 #endif
 
-extern struct folio *huge_zero_folio;
-extern unsigned long huge_zero_pfn;
 
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 static inline bool is_huge_zero_folio(const struct folio *folio)
-- 
2.49.0

Re: [PATCH v2 4/5] mm: add largest_zero_folio() routine

Posted by Lorenzo Stoakes 2 months, 3 weeks ago

Nit on subject - this is a function not a routine :)

On Mon, Jul 07, 2025 at 04:23:18PM +0200, Pankaj Raghav (Samsung) wrote:
> From: Pankaj Raghav <p.raghav@samsung.com>
>
> Add largest_zero_folio() routine so that huge_zero_folio can be
> used without the need to pass any mm struct. This will return ZERO_PAGE
> folio if CONFIG_STATIC_PMD_ZERO_PAGE is disabled or if we failed to
> allocate a PMD page from memblock.
>
> This routine can also be called even if THP is disabled.

This is pretty much implicit in the series as a whole though, so probably
not worth mentioning :)

>
> Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
> ---
>  include/linux/mm.h | 28 ++++++++++++++++++++++++++--
>  1 file changed, 26 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 428fe6d36b3c..d5543cf7b8e9 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -4018,17 +4018,41 @@ static inline bool vma_is_special_huge(const struct vm_area_struct *vma)
>
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE || CONFIG_HUGETLBFS */
>
> +extern struct folio *huge_zero_folio;
> +extern unsigned long huge_zero_pfn;

I mean this should be i huge_mm.h again imo, but at any rate I don't know why
you're moving them up here.

But maybe diff algorithm being weird?

> +
>  #ifdef CONFIG_STATIC_PMD_ZERO_PAGE
>  extern void __init static_pmd_zero_init(void);
> +
> +/*
> + * largest_zero_folio - Get the largest zero size folio available
> + *
> + * This function will return a PMD sized zero folio if CONFIG_STATIC_PMD_ZERO_PAGE
> + * is enabled. Otherwise, a ZERO_PAGE folio is returned.
> + *
> + * Deduce the size of the folio with folio_size instead of assuming the
> + * folio size.
> + */
> +static inline struct folio *largest_zero_folio(void)
> +{
> +	if(!huge_zero_folio)
> +		return page_folio(ZERO_PAGE(0));
> +
> +	return READ_ONCE(huge_zero_folio);
> +}
> +
>  #else
>  static inline void __init static_pmd_zero_init(void)
>  {
>  	return;

No need to return in void functions.

>  }
> +
> +static inline struct folio *largest_zero_folio(void)
> +{
> +	return page_folio(ZERO_PAGE(0));
> +}
>  #endif
>
> -extern struct folio *huge_zero_folio;
> -extern unsigned long huge_zero_pfn;
>
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>  static inline bool is_huge_zero_folio(const struct folio *folio)
> --
> 2.49.0
>

Re: [PATCH v2 4/5] mm: add largest_zero_folio() routine

Posted by David Hildenbrand 2 months, 3 weeks ago

On 07.07.25 16:23, Pankaj Raghav (Samsung) wrote:
> From: Pankaj Raghav <p.raghav@samsung.com>
> 
> Add largest_zero_folio() routine so that huge_zero_folio can be
> used without the need to pass any mm struct. This will return ZERO_PAGE
> folio if CONFIG_STATIC_PMD_ZERO_PAGE is disabled or if we failed to
> allocate a PMD page from memblock.
> 
> This routine can also be called even if THP is disabled.
> 
> Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
> ---
>   include/linux/mm.h | 28 ++++++++++++++++++++++++++--
>   1 file changed, 26 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 428fe6d36b3c..d5543cf7b8e9 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -4018,17 +4018,41 @@ static inline bool vma_is_special_huge(const struct vm_area_struct *vma)
>   
>   #endif /* CONFIG_TRANSPARENT_HUGEPAGE || CONFIG_HUGETLBFS */
>   
> +extern struct folio *huge_zero_folio;
> +extern unsigned long huge_zero_pfn;

No need for "extern".

> +
>   #ifdef CONFIG_STATIC_PMD_ZERO_PAGE
>   extern void __init static_pmd_zero_init(void);
> +
> +/*
> + * largest_zero_folio - Get the largest zero size folio available
> + *
> + * This function will return a PMD sized zero folio if CONFIG_STATIC_PMD_ZERO_PAGE
> + * is enabled. Otherwise, a ZERO_PAGE folio is returned.
> + *
> + * Deduce the size of the folio with folio_size instead of assuming the
> + * folio size.
> + */
> +static inline struct folio *largest_zero_folio(void)
> +{
> +	if(!huge_zero_folio)

Nit: if (!huge_zero_folio), but see below

I assume this check is only in place to handle whether static allocation 
failed, correct?

> +		return page_folio(ZERO_PAGE(0));
> +
> +	return READ_ONCE(huge_zero_folio);

READ_ONCE should not be required if it cannot get freed.

> +}
> +
>   #else
>   static inline void __init static_pmd_zero_init(void)
>   {
>   	return;
>   }
> +
> +static inline struct folio *largest_zero_folio(void)
> +{
> +	return page_folio(ZERO_PAGE(0));
> +}
>   #endif

Could we do:

static inline struct folio *largest_zero_folio(void)
{
	if (IS_ENABLED(CONFIG_STATIC_PMD_ZERO_PAGE) &&
  	    likely(huge_zero_folio))
		return huge_zero_folio;
	return page_folio(ZERO_PAGE(0));
}
	

-- 
Cheers,

David / dhildenb

Re: [PATCH v2 4/5] mm: add largest_zero_folio() routine

Posted by David Hildenbrand 2 months, 3 weeks ago

On 15.07.25 16:16, David Hildenbrand wrote:
> On 07.07.25 16:23, Pankaj Raghav (Samsung) wrote:
>> From: Pankaj Raghav <p.raghav@samsung.com>
>>
>> Add largest_zero_folio() routine so that huge_zero_folio can be
>> used without the need to pass any mm struct. This will return ZERO_PAGE
>> folio if CONFIG_STATIC_PMD_ZERO_PAGE is disabled or if we failed to
>> allocate a PMD page from memblock.
>>
>> This routine can also be called even if THP is disabled.
>>
>> Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
>> ---
>>    include/linux/mm.h | 28 ++++++++++++++++++++++++++--
>>    1 file changed, 26 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 428fe6d36b3c..d5543cf7b8e9 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -4018,17 +4018,41 @@ static inline bool vma_is_special_huge(const struct vm_area_struct *vma)
>>    
>>    #endif /* CONFIG_TRANSPARENT_HUGEPAGE || CONFIG_HUGETLBFS */
>>    
>> +extern struct folio *huge_zero_folio;
>> +extern unsigned long huge_zero_pfn;
> 
> No need for "extern".

Scratch that, was confused with functions ...

-- 
Cheers,

David / dhildenb