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

Pankaj Raghav (Samsung) posted 5 patches 6 months, 1 week ago
There is a newer version of this series
[PATCH 4/5] mm: add largest_zero_folio() routine
Posted by Pankaj Raghav (Samsung) 6 months, 1 week ago 
From: Pankaj Raghav <p.raghav@samsung.com>

Add largest_zero_folio() routine so that huge_zero_folio can be
used directly when CONFIG_STATIC_HUGE_ZERO_FOLIO is enabled. This will
return ZERO_PAGE folio if CONFIG_STATIC_HUGE_ZERO_FOLIO is disabled or
if we failed to allocate a huge_zero_folio.

Co-developed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
---
 include/linux/huge_mm.h | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 78ebceb61d0e..c44a6736704b 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -716,4 +716,21 @@ static inline int split_folio_to_order(struct folio *folio, int new_order)
 	return split_folio_to_list_to_order(folio, NULL, new_order);
 }
 
+/*
+ * largest_zero_folio - Get the largest zero size folio available
+ *
+ * This function will return huge_zero_folio if CONFIG_STATIC_HUGE_ZERO_FOLIO
+ * 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)
+{
+	struct folio *folio = get_static_huge_zero_folio();
+
+	if (folio)
+		return folio;
+	return page_folio(ZERO_PAGE(0));
+}
 #endif /* _LINUX_HUGE_MM_H */
-- 
2.49.0
Re: [PATCH 4/5] mm: add largest_zero_folio() routine
Posted by Lorenzo Stoakes 6 months, 1 week ago 
On Mon, Aug 04, 2025 at 02:13:55PM +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 directly when CONFIG_STATIC_HUGE_ZERO_FOLIO is enabled. This will
> return ZERO_PAGE folio if CONFIG_STATIC_HUGE_ZERO_FOLIO is disabled or
> if we failed to allocate a huge_zero_folio.
>
> Co-developed-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>

This looks good to me minus nit + comment below, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> ---
>  include/linux/huge_mm.h | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 78ebceb61d0e..c44a6736704b 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -716,4 +716,21 @@ static inline int split_folio_to_order(struct folio *folio, int new_order)
>  	return split_folio_to_list_to_order(folio, NULL, new_order);
>  }
>
> +/*

Maybe let's make this a kernel doc comment?

> + * largest_zero_folio - Get the largest zero size folio available
> + *
> + * This function will return huge_zero_folio if CONFIG_STATIC_HUGE_ZERO_FOLIO
> + * 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)
> +{
> +	struct folio *folio = get_static_huge_zero_folio();
> +
> +	if (folio)
> +		return folio;
> +	return page_folio(ZERO_PAGE(0));

Feels like we should have this in a wrapper function somewhere. Then again it
seems people generally always open-code 'ZERO_PAGE(0)' anyway so maybe this is
sort of the 'way things are done'? :P

> +}
>  #endif /* _LINUX_HUGE_MM_H */
> --
> 2.49.0
>
Re: [PATCH 4/5] mm: add largest_zero_folio() routine
Posted by Pankaj Raghav (Samsung) 6 months, 1 week ago 
On Mon, Aug 04, 2025 at 05:50:46PM +0100, Lorenzo Stoakes wrote:
> On Mon, Aug 04, 2025 at 02:13:55PM +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 directly when CONFIG_STATIC_HUGE_ZERO_FOLIO is enabled. This will
> > return ZERO_PAGE folio if CONFIG_STATIC_HUGE_ZERO_FOLIO is disabled or
> > if we failed to allocate a huge_zero_folio.
> >
> > Co-developed-by: David Hildenbrand <david@redhat.com>
> > Signed-off-by: David Hildenbrand <david@redhat.com>
> > Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
> 
> This looks good to me minus nit + comment below, so:
> 
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> 
Thanks.
> > ---
> >  include/linux/huge_mm.h | 17 +++++++++++++++++
> >  1 file changed, 17 insertions(+)
> >
> > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > index 78ebceb61d0e..c44a6736704b 100644
> > --- a/include/linux/huge_mm.h
> > +++ b/include/linux/huge_mm.h
> > @@ -716,4 +716,21 @@ static inline int split_folio_to_order(struct folio *folio, int new_order)
> >  	return split_folio_to_list_to_order(folio, NULL, new_order);
> >  }
> >
> > +/*
> 
> Maybe let's make this a kernel doc comment?
Yes, I will change it in the next version :)
> 
> > + * largest_zero_folio - Get the largest zero size folio available
> > + *
> > + * This function will return huge_zero_folio if CONFIG_STATIC_HUGE_ZERO_FOLIO
> > + * 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)
> > +{
> > +	struct folio *folio = get_static_huge_zero_folio();
> > +
> > +	if (folio)
> > +		return folio;
> > +	return page_folio(ZERO_PAGE(0));
> 
> Feels like we should have this in a wrapper function somewhere. Then again it
> seems people generally always open-code 'ZERO_PAGE(0)' anyway so maybe this is
> sort of the 'way things are done'? :P
> 
> > +}
> >  #endif /* _LINUX_HUGE_MM_H */
> > --
> > 2.49.0
> >
Re: [PATCH 4/5] mm: add largest_zero_folio() routine
Posted by Zi Yan 6 months, 1 week ago 
On 4 Aug 2025, at 8:13, Pankaj Raghav (Samsung) wrote:

> From: Pankaj Raghav <p.raghav@samsung.com>
>
> Add largest_zero_folio() routine so that huge_zero_folio can be
> used directly when CONFIG_STATIC_HUGE_ZERO_FOLIO is enabled. This will
> return ZERO_PAGE folio if CONFIG_STATIC_HUGE_ZERO_FOLIO is disabled or
> if we failed to allocate a huge_zero_folio.
>
> Co-developed-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
> ---
>  include/linux/huge_mm.h | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
>
Reviewed-by: Zi Yan <ziy@nvidia.com>

Best Regards,
Yan, Zi
Re: [PATCH 4/5] mm: add largest_zero_folio() routine
Posted by Dave Hansen 6 months, 1 week ago 
On 8/4/25 05:13, Pankaj Raghav (Samsung) wrote:
> From: Pankaj Raghav <p.raghav@samsung.com>
> 
> Add largest_zero_folio() routine so that huge_zero_folio can be
> used directly when CONFIG_STATIC_HUGE_ZERO_FOLIO is enabled. This will
> return ZERO_PAGE folio if CONFIG_STATIC_HUGE_ZERO_FOLIO is disabled or
> if we failed to allocate a huge_zero_folio.

This changelog is telling a lot of the "what" but none of the "why".

The existing huge zero folio API is for users that have an mm. This is
*only* for folks that want a huge zero folio but don't have an mm.
That's _why_ this function is needed. It's in this series because there
was no way to get a huge zero folio before the permanent one was
introduced in this series.

Can we please get some of that into the function comment and changelog?
It's critical.

> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 78ebceb61d0e..c44a6736704b 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -716,4 +716,21 @@ static inline int split_folio_to_order(struct folio *folio, int new_order)
>  	return split_folio_to_list_to_order(folio, NULL, new_order);
>  }
>  
> +/*
> + * largest_zero_folio - Get the largest zero size folio available
> + *
> + * This function will return huge_zero_folio if CONFIG_STATIC_HUGE_ZERO_FOLIO
> + * is enabled. Otherwise, a ZERO_PAGE folio is returned.
> + *
> + * Deduce the size of the folio with folio_size instead of assuming the
> + * folio size.
> + */

This comment needs to get fleshed out. It at _least_ needs to say
something along the lines of:

	Use this when a huge zero folio is needed but there is no mm
	lifetime to tie it to. Basically, use this when you can't use
	mm_get_huge_zero_folio().

> +static inline struct folio *largest_zero_folio(void)
> +{
> +	struct folio *folio = get_static_huge_zero_folio();
> +
> +	if (folio)
> +		return folio;

There needs to be a newline in here.

> +	return page_folio(ZERO_PAGE(0));
> +}
>  #endif /* _LINUX_HUGE_MM_H */

The code is fine, but the changelog and comments need quite a bit of work.
Re: [PATCH 4/5] mm: add largest_zero_folio() routine
Posted by Pankaj Raghav (Samsung) 6 months ago 
On Tue, Aug 05, 2025 at 09:42:14AM -0700, Dave Hansen wrote:
> On 8/4/25 05:13, Pankaj Raghav (Samsung) wrote:
> > From: Pankaj Raghav <p.raghav@samsung.com>
> > 
> > Add largest_zero_folio() routine so that huge_zero_folio can be
> > used directly when CONFIG_STATIC_HUGE_ZERO_FOLIO is enabled. This will
> > return ZERO_PAGE folio if CONFIG_STATIC_HUGE_ZERO_FOLIO is disabled or
> > if we failed to allocate a huge_zero_folio.
> 
> This changelog is telling a lot of the "what" but none of the "why".
> 
> The existing huge zero folio API is for users that have an mm. This is
> *only* for folks that want a huge zero folio but don't have an mm.
> That's _why_ this function is needed. It's in this series because there
> was no way to get a huge zero folio before the permanent one was
> introduced in this series.
> 
> Can we please get some of that into the function comment and changelog?
> It's critical.

Valid point. I will include that in the next version.

> 
> > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > index 78ebceb61d0e..c44a6736704b 100644
> > --- a/include/linux/huge_mm.h
> > +++ b/include/linux/huge_mm.h
> > @@ -716,4 +716,21 @@ static inline int split_folio_to_order(struct folio *folio, int new_order)
> >  	return split_folio_to_list_to_order(folio, NULL, new_order);
> >  }
> >  
> > +/*
> > + * largest_zero_folio - Get the largest zero size folio available
> > + *
> > + * This function will return huge_zero_folio if CONFIG_STATIC_HUGE_ZERO_FOLIO
> > + * is enabled. Otherwise, a ZERO_PAGE folio is returned.
> > + *
> > + * Deduce the size of the folio with folio_size instead of assuming the
> > + * folio size.
> > + */
> 
> This comment needs to get fleshed out. It at _least_ needs to say
> something along the lines of:
> 
> 	Use this when a huge zero folio is needed but there is no mm
> 	lifetime to tie it to. Basically, use this when you can't use
> 	mm_get_huge_zero_folio().
> 

Agreed.

> > +static inline struct folio *largest_zero_folio(void)
> > +{
> > +	struct folio *folio = get_static_huge_zero_folio();
> > +
> > +	if (folio)
> > +		return folio;
> 
> There needs to be a newline in here.
> 
> > +	return page_folio(ZERO_PAGE(0));
> > +}
> >  #endif /* _LINUX_HUGE_MM_H */
> 
> The code is fine, but the changelog and comments need quite a bit of work.

Sounds good to me. I will make the changes. Thanks Dave.

--
Pankaj

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