setting /transparent_hugepages/enabled=always allows applications
to benefit from THPs without having to madvise. However, the pf handler
takes very few considerations to decide weather or not to actually use a
THP. This can lead to a lot of wasted memory. khugepaged only operates
on memory that was either allocated with enabled=always or MADV_HUGEPAGE.

Introduce the ability to set enabled=defer, which will prevent THPs from
being allocated by the page fault handler unless madvise is set,
leaving it up to khugepaged to decide which allocations will collapse to a
THP. This should allow applications to benefits from THPs, while curbing
some of the memory waste.

Signed-off-by: Nico Pache <npache@redhat.com>
---
 include/linux/huge_mm.h | 15 +++++++++++++--
 mm/huge_memory.c        | 31 +++++++++++++++++++++++++++----
 2 files changed, 40 insertions(+), 6 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 93e509b6c00e..fb381ca720ea 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -44,6 +44,7 @@ enum transparent_hugepage_flag {
 	TRANSPARENT_HUGEPAGE_UNSUPPORTED,
 	TRANSPARENT_HUGEPAGE_FLAG,
 	TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
+	TRANSPARENT_HUGEPAGE_DEFER_PF_INST_FLAG,
 	TRANSPARENT_HUGEPAGE_DEFRAG_DIRECT_FLAG,
 	TRANSPARENT_HUGEPAGE_DEFRAG_KSWAPD_FLAG,
 	TRANSPARENT_HUGEPAGE_DEFRAG_KSWAPD_OR_MADV_FLAG,
@@ -177,6 +178,7 @@ static inline bool hugepage_global_enabled(void)
 {
 	return transparent_hugepage_flags &
 			((1<<TRANSPARENT_HUGEPAGE_FLAG) |
+			(1<<TRANSPARENT_HUGEPAGE_DEFER_PF_INST_FLAG) |
 			(1<<TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG));
 }
 
@@ -186,6 +188,12 @@ static inline bool hugepage_global_always(void)
 			(1<<TRANSPARENT_HUGEPAGE_FLAG);
 }
 
+static inline bool hugepage_global_defer(void)
+{
+	return transparent_hugepage_flags &
+			(1<<TRANSPARENT_HUGEPAGE_DEFER_PF_INST_FLAG);
+}
+
 static inline int highest_order(unsigned long orders)
 {
 	return fls_long(orders) - 1;
@@ -282,13 +290,16 @@ unsigned long thp_vma_allowable_orders(struct vm_area_struct *vma,
 				       unsigned long tva_flags,
 				       unsigned long orders)
 {
+	if ((tva_flags & TVA_IN_PF) && hugepage_global_defer() &&
+			!(vm_flags & VM_HUGEPAGE))
+		return 0;
+
 	/* Optimization to check if required orders are enabled early. */
 	if ((tva_flags & TVA_ENFORCE_SYSFS) && vma_is_anonymous(vma)) {
 		unsigned long mask = READ_ONCE(huge_anon_orders_always);
-
 		if (vm_flags & VM_HUGEPAGE)
 			mask |= READ_ONCE(huge_anon_orders_madvise);
-		if (hugepage_global_always() ||
+		if (hugepage_global_always() || hugepage_global_defer() ||
 		    ((vm_flags & VM_HUGEPAGE) && hugepage_global_enabled()))
 			mask |= READ_ONCE(huge_anon_orders_inherit);
 
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 3d3ebdc002d5..a5e66a12bae8 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -297,12 +297,15 @@ static ssize_t enabled_show(struct kobject *kobj,
 	const char *output;
 
 	if (test_bit(TRANSPARENT_HUGEPAGE_FLAG, &transparent_hugepage_flags))
-		output = "[always] madvise never";
+		output = "[always] madvise defer never";
 	else if (test_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
 			  &transparent_hugepage_flags))
-		output = "always [madvise] never";
+		output = "always [madvise] defer never";
+	else if (test_bit(TRANSPARENT_HUGEPAGE_DEFER_PF_INST_FLAG,
+			  &transparent_hugepage_flags))
+		output = "always madvise [defer] never";
 	else
-		output = "always madvise [never]";
+		output = "always madvise defer [never]";
 
 	return sysfs_emit(buf, "%s\n", output);
 }
@@ -315,13 +318,20 @@ static ssize_t enabled_store(struct kobject *kobj,
 
 	if (sysfs_streq(buf, "always")) {
 		clear_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG, &transparent_hugepage_flags);
+		clear_bit(TRANSPARENT_HUGEPAGE_DEFER_PF_INST_FLAG, &transparent_hugepage_flags);
 		set_bit(TRANSPARENT_HUGEPAGE_FLAG, &transparent_hugepage_flags);
+	} else if (sysfs_streq(buf, "defer")) {
+		clear_bit(TRANSPARENT_HUGEPAGE_FLAG, &transparent_hugepage_flags);
+		clear_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG, &transparent_hugepage_flags);
+		set_bit(TRANSPARENT_HUGEPAGE_DEFER_PF_INST_FLAG, &transparent_hugepage_flags);
 	} else if (sysfs_streq(buf, "madvise")) {
 		clear_bit(TRANSPARENT_HUGEPAGE_FLAG, &transparent_hugepage_flags);
+		clear_bit(TRANSPARENT_HUGEPAGE_DEFER_PF_INST_FLAG, &transparent_hugepage_flags);
 		set_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG, &transparent_hugepage_flags);
 	} else if (sysfs_streq(buf, "never")) {
 		clear_bit(TRANSPARENT_HUGEPAGE_FLAG, &transparent_hugepage_flags);
 		clear_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG, &transparent_hugepage_flags);
+		clear_bit(TRANSPARENT_HUGEPAGE_DEFER_PF_INST_FLAG, &transparent_hugepage_flags);
 	} else
 		ret = -EINVAL;
 
@@ -943,18 +953,31 @@ static int __init setup_transparent_hugepage(char *str)
 			&transparent_hugepage_flags);
 		clear_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
 			  &transparent_hugepage_flags);
+		clear_bit(TRANSPARENT_HUGEPAGE_DEFER_PF_INST_FLAG,
+			  &transparent_hugepage_flags);
 		ret = 1;
+	} else if (!strcmp(str, "defer")) {
+		clear_bit(TRANSPARENT_HUGEPAGE_FLAG,
+			  &transparent_hugepage_flags);
+		clear_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
+			  &transparent_hugepage_flags);
+		set_bit(TRANSPARENT_HUGEPAGE_DEFER_PF_INST_FLAG,
+			  &transparent_hugepage_flags);
 	} else if (!strcmp(str, "madvise")) {
 		clear_bit(TRANSPARENT_HUGEPAGE_FLAG,
 			  &transparent_hugepage_flags);
+		clear_bit(TRANSPARENT_HUGEPAGE_DEFER_PF_INST_FLAG,
+			  &transparent_hugepage_flags);
 		set_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
-			&transparent_hugepage_flags);
+			  &transparent_hugepage_flags);
 		ret = 1;
 	} else if (!strcmp(str, "never")) {
 		clear_bit(TRANSPARENT_HUGEPAGE_FLAG,
 			  &transparent_hugepage_flags);
 		clear_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
 			  &transparent_hugepage_flags);
+		clear_bit(TRANSPARENT_HUGEPAGE_DEFER_PF_INST_FLAG,
+			  &transparent_hugepage_flags);
 		ret = 1;
 	}
 out:
-- 
2.48.1

On 11/02/2025 00:40, Nico Pache wrote:
> setting /transparent_hugepages/enabled=always allows applications
> to benefit from THPs without having to madvise. However, the pf handler
> takes very few considerations to decide weather or not to actually use a
> THP. This can lead to a lot of wasted memory. khugepaged only operates
> on memory that was either allocated with enabled=always or MADV_HUGEPAGE.
> 
> Introduce the ability to set enabled=defer, which will prevent THPs from
> being allocated by the page fault handler unless madvise is set,
> leaving it up to khugepaged to decide which allocations will collapse to a
> THP. This should allow applications to benefits from THPs, while curbing
> some of the memory waste.
> 
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
>  include/linux/huge_mm.h | 15 +++++++++++++--
>  mm/huge_memory.c        | 31 +++++++++++++++++++++++++++----
>  2 files changed, 40 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 93e509b6c00e..fb381ca720ea 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -44,6 +44,7 @@ enum transparent_hugepage_flag {
>  	TRANSPARENT_HUGEPAGE_UNSUPPORTED,
>  	TRANSPARENT_HUGEPAGE_FLAG,
>  	TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
> +	TRANSPARENT_HUGEPAGE_DEFER_PF_INST_FLAG,

No strong preference, but maybe just TRANSPARENT_HUGEPAGE_DEFER_FLAG might be better?

On Mon, Feb 17, 2025 at 7:59 AM Usama Arif <usamaarif642@gmail.com> wrote:
>
>
>
> On 11/02/2025 00:40, Nico Pache wrote:
> > setting /transparent_hugepages/enabled=always allows applications
> > to benefit from THPs without having to madvise. However, the pf handler
> > takes very few considerations to decide weather or not to actually use a
> > THP. This can lead to a lot of wasted memory. khugepaged only operates
> > on memory that was either allocated with enabled=always or MADV_HUGEPAGE.
> >
> > Introduce the ability to set enabled=defer, which will prevent THPs from
> > being allocated by the page fault handler unless madvise is set,
> > leaving it up to khugepaged to decide which allocations will collapse to a
> > THP. This should allow applications to benefits from THPs, while curbing
> > some of the memory waste.
> >
> > Signed-off-by: Nico Pache <npache@redhat.com>
> > ---
> >  include/linux/huge_mm.h | 15 +++++++++++++--
> >  mm/huge_memory.c        | 31 +++++++++++++++++++++++++++----
> >  2 files changed, 40 insertions(+), 6 deletions(-)
> >
> > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > index 93e509b6c00e..fb381ca720ea 100644
> > --- a/include/linux/huge_mm.h
> > +++ b/include/linux/huge_mm.h
> > @@ -44,6 +44,7 @@ enum transparent_hugepage_flag {
> >       TRANSPARENT_HUGEPAGE_UNSUPPORTED,
> >       TRANSPARENT_HUGEPAGE_FLAG,
> >       TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
> > +     TRANSPARENT_HUGEPAGE_DEFER_PF_INST_FLAG,
>
> No strong preference, but maybe just TRANSPARENT_HUGEPAGE_DEFER_FLAG might be better?

Not a bad idea, TRANSPARENT_HUGEPAGE_DEFER_PF_INST_FLAG is pretty long!
>