[PATCH v4 0/5] Allow order zero pages in page reporting

[PATCH v4 0/5] Allow order zero pages in page reporting

[Yuvraj Sakshith]

Today, page reporting sets page_reporting_order in two ways:

(1) page_reporting.page_reporting_order cmdline parameter
(2) Driver can pass order while registering itself.

In both cases, order zero is ignored by free page reporting
because it is used to set page_reporting_order to a default
value, like MAX_PAGE_ORDER.

In some cases we might want page_reporting_order to be zero.

For instance, when virtio-balloon runs inside a guest with
tiny memory (say, 16MB), it might not be able to find a order 1 page
(or in the worst case order MAX_PAGE_ORDER page) after some uptime.
Page reporting should be able to return order zero pages back for
optimal memory relinquishment.

This patch changes the default fallback value from '0' to '-1' in
all possible clients of free page reporting (hv_balloon and
virtio-balloon) together with allowing '0' as a valid order in
page_reporting_register().

Changes in v1:
- Introduce PAGE_REPORTING_DEFAULT_ORDER macro (initially set to 0).
- Make use of new macro in drivers (hv_balloon and virtio-balloon)
        working with page reporting.
- Change PAGE_REPORTING_DEFAULT_ORDER to -1 as zero is a valid
        page order that can be requested.

Changes in v2:
- Better naming. Replace PAGE_REPORTING_DEFAULT_ORDER with
        PAGE_REPORTING_ORDER_UNSPECIFIED. This takes care of
        the situation where page reporting order is not specified
        in the commandline.
- Minor commit message changes.

Changes in v3:
- Setting page_reporting_order's initial value to
	PAGE_REPORTING_ORDER_UNSPECIFIED moved to
	PATCH #5.

Changes in v4:
- Move PAGE_REPORTING_ORDER_UNSPECIFIED's usage with
	page_reporting_order to patch #5.

Yuvraj Sakshith (5):
  mm/page_reporting: add PAGE_REPORTING_ORDER_UNSPECIFIED
  virtio_balloon: set unspecified page reporting order
  hv_balloon: set unspecified page reporting order
  mm/page_reporting: change PAGE_REPORTING_ORDER_UNSPECIFIED to -1
  mm/page_reporting: change page_reporting_order to
    PAGE_REPORTING_ORDER_UNSPECIFIED

 drivers/hv/hv_balloon.c         | 2 +-
 drivers/virtio/virtio_balloon.c | 2 ++
 include/linux/page_reporting.h  | 1 +
 mm/page_reporting.c             | 7 ++++---
 4 files changed, 8 insertions(+), 4 deletions(-)

-- 
2.34.1

[PATCH v4 1/5] mm/page_reporting: add PAGE_REPORTING_ORDER_UNSPECIFIED

[Yuvraj Sakshith]

Drivers can pass order of pages to be reported while
registering itself. Today, this is a magic number, 0.

Label this with PAGE_REPORTING_ORDER_UNSPECIFIED and
check for it when the driver is being registered.

This macro will be used in relevant drivers next.

Signed-off-by: Yuvraj Sakshith <yuvraj.sakshith@oss.qualcomm.com>
---
 include/linux/page_reporting.h | 1 +
 mm/page_reporting.c            | 3 ++-
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/linux/page_reporting.h b/include/linux/page_reporting.h
index fe648dfa3..d1886c657 100644
--- a/include/linux/page_reporting.h
+++ b/include/linux/page_reporting.h
@@ -7,6 +7,7 @@
 
 /* This value should always be a power of 2, see page_reporting_cycle() */
 #define PAGE_REPORTING_CAPACITY		32
+#define PAGE_REPORTING_ORDER_UNSPECIFIED	0
 
 struct page_reporting_dev_info {
 	/* function that alters pages to make them "reported" */
diff --git a/mm/page_reporting.c b/mm/page_reporting.c
index e4c428e61..a97ee07cb 100644
--- a/mm/page_reporting.c
+++ b/mm/page_reporting.c
@@ -370,7 +370,8 @@ int page_reporting_register(struct page_reporting_dev_info *prdev)
 	 */
 
 	if (page_reporting_order == -1) {
-		if (prdev->order > 0 && prdev->order <= MAX_PAGE_ORDER)
+		if (prdev->order != PAGE_REPORTING_ORDER_UNSPECIFIED &&
+			prdev->order <= MAX_PAGE_ORDER)
 			page_reporting_order = prdev->order;
 		else
 			page_reporting_order = pageblock_order;
-- 
2.34.1

Re: [PATCH v4 1/5] mm/page_reporting: add PAGE_REPORTING_ORDER_UNSPECIFIED

[David Hildenbrand (Arm)]

On 3/3/26 12:30, Yuvraj Sakshith wrote:
> Drivers can pass order of pages to be reported while
> registering itself. Today, this is a magic number, 0.
> 
> Label this with PAGE_REPORTING_ORDER_UNSPECIFIED and
> check for it when the driver is being registered.
> 
> This macro will be used in relevant drivers next.
> 
> Signed-off-by: Yuvraj Sakshith <yuvraj.sakshith@oss.qualcomm.com>
> ---
>  include/linux/page_reporting.h | 1 +
>  mm/page_reporting.c            | 3 ++-
>  2 files changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/page_reporting.h b/include/linux/page_reporting.h
> index fe648dfa3..d1886c657 100644
> --- a/include/linux/page_reporting.h
> +++ b/include/linux/page_reporting.h
> @@ -7,6 +7,7 @@
>  
>  /* This value should always be a power of 2, see page_reporting_cycle() */
>  #define PAGE_REPORTING_CAPACITY		32
> +#define PAGE_REPORTING_ORDER_UNSPECIFIED	0
>  
>  struct page_reporting_dev_info {
>  	/* function that alters pages to make them "reported" */
> diff --git a/mm/page_reporting.c b/mm/page_reporting.c
> index e4c428e61..a97ee07cb 100644
> --- a/mm/page_reporting.c
> +++ b/mm/page_reporting.c
> @@ -370,7 +370,8 @@ int page_reporting_register(struct page_reporting_dev_info *prdev)
>  	 */
>  
>  	if (page_reporting_order == -1) {
> -		if (prdev->order > 0 && prdev->order <= MAX_PAGE_ORDER)
> +		if (prdev->order != PAGE_REPORTING_ORDER_UNSPECIFIED &&
> +			prdev->order <= MAX_PAGE_ORDER)

Nit:

	if (prdev->order != PAGE_REPORTING_ORDER_UNSPECIFIED &&
	    prdev->order <= MAX_PAGE_ORDER)

	    ^ aligned here

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David

[PATCH v4 2/5] virtio_balloon: set unspecified page reporting order

[Yuvraj Sakshith]

virtio_balloon page reporting order is set to MAX_PAGE_ORDER implicitly
as vb->prdev.order is never initialised and is auto-set to zero.

Explicitly mention usage of default page order by making use of
PAGE_REPORTING_ORDER_UNSPECIFIED fallback value.

Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Yuvraj Sakshith <yuvraj.sakshith@oss.qualcomm.com>
---
 drivers/virtio/virtio_balloon.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 74fe59f5a..2dfe2bcd8 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -1044,6 +1044,8 @@ static int virtballoon_probe(struct virtio_device *vdev)
 			goto out_unregister_oom;
 		}
 
+		vb->pr_dev_info.order = PAGE_REPORTING_ORDER_UNSPECIFIED;
+
 		/*
 		 * The default page reporting order is @pageblock_order, which
 		 * corresponds to 512MB in size on ARM64 when 64KB base page
-- 
2.34.1

[PATCH v4 3/5] hv_balloon: set unspecified page reporting order

[Yuvraj Sakshith]

Explicitly mention page reporting order to be set to
default value using PAGE_REPORTING_ORDER_UNSPECIFIED fallback
value.

Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Yuvraj Sakshith <yuvraj.sakshith@oss.qualcomm.com>
---
 drivers/hv/hv_balloon.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
index 2b4080e51..09da68101 100644
--- a/drivers/hv/hv_balloon.c
+++ b/drivers/hv/hv_balloon.c
@@ -1663,7 +1663,7 @@ static void enable_page_reporting(void)
 	 * We let the page_reporting_order parameter decide the order
 	 * in the page_reporting code
 	 */
-	dm_device.pr_dev_info.order = 0;
+	dm_device.pr_dev_info.order = PAGE_REPORTING_ORDER_UNSPECIFIED;
 	ret = page_reporting_register(&dm_device.pr_dev_info);
 	if (ret < 0) {
 		dm_device.pr_dev_info.report = NULL;
-- 
2.34.1

[PATCH v4 4/5] mm/page_reporting: change PAGE_REPORTING_ORDER_UNSPECIFIED to -1

[Yuvraj Sakshith]

PAGE_REPORTING_ORDER_UNSPECIFIED is now set to zero. This means,
pages of order zero cannot be reported to a client/driver -- as zero
is used to signal a fallback to MAX_PAGE_ORDER.

Change PAGE_REPORTING_ORDER_UNSPECIFIED to (-1),
so that zero can be used as a valid order with which pages can
be reported.

Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Yuvraj Sakshith <yuvraj.sakshith@oss.qualcomm.com>
---
 include/linux/page_reporting.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/page_reporting.h b/include/linux/page_reporting.h
index d1886c657..9d4ca5c21 100644
--- a/include/linux/page_reporting.h
+++ b/include/linux/page_reporting.h
@@ -7,7 +7,7 @@
 
 /* This value should always be a power of 2, see page_reporting_cycle() */
 #define PAGE_REPORTING_CAPACITY		32
-#define PAGE_REPORTING_ORDER_UNSPECIFIED	0
+#define PAGE_REPORTING_ORDER_UNSPECIFIED	-1
 
 struct page_reporting_dev_info {
 	/* function that alters pages to make them "reported" */
-- 
2.34.1

[PATCH v4 5/5] mm/page_reporting: change page_reporting_order to PAGE_REPORTING_ORDER_UNSPECIFIED

[Yuvraj Sakshith]

page_reporting_order when uninitialised, holds a magic number -1.

Since we now maintain PAGE_REPORTING_ORDER_UNSPECIFIED as -1, which
is also a flag, set page_reporting_order to this flag.

Signed-off-by: Yuvraj Sakshith <yuvraj.sakshith@oss.qualcomm.com>
---
 mm/page_reporting.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/page_reporting.c b/mm/page_reporting.c
index a97ee07cb..21c11b75e 100644
--- a/mm/page_reporting.c
+++ b/mm/page_reporting.c
@@ -12,7 +12,7 @@
 #include "internal.h"
 
 /* Initialize to an unsupported value */
-unsigned int page_reporting_order = -1;
+unsigned int page_reporting_order = PAGE_REPORTING_ORDER_UNSPECIFIED;
 
 static int page_order_update_notify(const char *val, const struct kernel_param *kp)
 {
@@ -369,7 +369,7 @@ int page_reporting_register(struct page_reporting_dev_info *prdev)
 	 * pageblock_order.
 	 */
 
-	if (page_reporting_order == -1) {
+	if (page_reporting_order == PAGE_REPORTING_ORDER_UNSPECIFIED) {
 		if (prdev->order != PAGE_REPORTING_ORDER_UNSPECIFIED &&
 			prdev->order <= MAX_PAGE_ORDER)
 			page_reporting_order = prdev->order;
-- 
2.34.1

Re: [PATCH v4 5/5] mm/page_reporting: change page_reporting_order to PAGE_REPORTING_ORDER_UNSPECIFIED

[David Hildenbrand (Arm)]

On 3/3/26 12:30, Yuvraj Sakshith wrote:
> page_reporting_order when uninitialised, holds a magic number -1.
> 
> Since we now maintain PAGE_REPORTING_ORDER_UNSPECIFIED as -1, which
> is also a flag, set page_reporting_order to this flag.

It's not really a flag. It's a handle for a magic value :)

> 
> Signed-off-by: Yuvraj Sakshith <yuvraj.sakshith@oss.qualcomm.com>
> ---
>  mm/page_reporting.c | 4 ++--

Thanks!

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David

RE: [PATCH v4 0/5] Allow order zero pages in page reporting

[Michael Kelley]

From: Yuvraj Sakshith <yuvraj.sakshith@oss.qualcomm.com> Sent: Tuesday, March 3, 2026 3:30 AM
> 
> Today, page reporting sets page_reporting_order in two ways:
> 
> (1) page_reporting.page_reporting_order cmdline parameter
> (2) Driver can pass order while registering itself.
> 
> In both cases, order zero is ignored by free page reporting
> because it is used to set page_reporting_order to a default
> value, like MAX_PAGE_ORDER.
> 
> In some cases we might want page_reporting_order to be zero.
> 
> For instance, when virtio-balloon runs inside a guest with
> tiny memory (say, 16MB), it might not be able to find a order 1 page
> (or in the worst case order MAX_PAGE_ORDER page) after some uptime.
> Page reporting should be able to return order zero pages back for
> optimal memory relinquishment.
> 
> This patch changes the default fallback value from '0' to '-1' in
> all possible clients of free page reporting (hv_balloon and
> virtio-balloon) together with allowing '0' as a valid order in
> page_reporting_register().
> 
> Changes in v1:
> - Introduce PAGE_REPORTING_DEFAULT_ORDER macro (initially set to 0).
> - Make use of new macro in drivers (hv_balloon and virtio-balloon)
>         working with page reporting.
> - Change PAGE_REPORTING_DEFAULT_ORDER to -1 as zero is a valid
>         page order that can be requested.
> 
> Changes in v2:
> - Better naming. Replace PAGE_REPORTING_DEFAULT_ORDER with
>         PAGE_REPORTING_ORDER_UNSPECIFIED. This takes care of
>         the situation where page reporting order is not specified
>         in the commandline.
> - Minor commit message changes.
> 
> Changes in v3:
> - Setting page_reporting_order's initial value to
> 	PAGE_REPORTING_ORDER_UNSPECIFIED moved to
> 	PATCH #5.
> 
> Changes in v4:
> - Move PAGE_REPORTING_ORDER_UNSPECIFIED's usage with
> 	page_reporting_order to patch #5.
> 
> Yuvraj Sakshith (5):
>   mm/page_reporting: add PAGE_REPORTING_ORDER_UNSPECIFIED
>   virtio_balloon: set unspecified page reporting order
>   hv_balloon: set unspecified page reporting order
>   mm/page_reporting: change PAGE_REPORTING_ORDER_UNSPECIFIED to -1
>   mm/page_reporting: change page_reporting_order to
>     PAGE_REPORTING_ORDER_UNSPECIFIED
> 
>  drivers/hv/hv_balloon.c         | 2 +-
>  drivers/virtio/virtio_balloon.c | 2 ++
>  include/linux/page_reporting.h  | 1 +
>  mm/page_reporting.c             | 7 ++++---
>  4 files changed, 8 insertions(+), 4 deletions(-)
> 
> --
> 2.34.1
> 

For the entire series,

Reviewed-by: Michael Kelley <mhklinux@outlook.com>

Re: [PATCH v4 0/5] Allow order zero pages in page reporting

[Michael S. Tsirkin]

On Tue, Mar 03, 2026 at 03:30:27AM -0800, Yuvraj Sakshith wrote:
> Today, page reporting sets page_reporting_order in two ways:
> 
> (1) page_reporting.page_reporting_order cmdline parameter
> (2) Driver can pass order while registering itself.
> 
> In both cases, order zero is ignored by free page reporting
> because it is used to set page_reporting_order to a default
> value, like MAX_PAGE_ORDER.
> 
> In some cases we might want page_reporting_order to be zero.
> 
> For instance, when virtio-balloon runs inside a guest with
> tiny memory (say, 16MB), it might not be able to find a order 1 page
> (or in the worst case order MAX_PAGE_ORDER page) after some uptime.
> Page reporting should be able to return order zero pages back for
> optimal memory relinquishment.
> 
> This patch changes the default fallback value from '0' to '-1' in
> all possible clients of free page reporting (hv_balloon and
> virtio-balloon) together with allowing '0' as a valid order in
> page_reporting_register().

virtio change is a technicality, so

Acked-by: Michael S. Tsirkin <mst@redhat.com>


> Changes in v1:
> - Introduce PAGE_REPORTING_DEFAULT_ORDER macro (initially set to 0).
> - Make use of new macro in drivers (hv_balloon and virtio-balloon)
>         working with page reporting.
> - Change PAGE_REPORTING_DEFAULT_ORDER to -1 as zero is a valid
>         page order that can be requested.
> 
> Changes in v2:
> - Better naming. Replace PAGE_REPORTING_DEFAULT_ORDER with
>         PAGE_REPORTING_ORDER_UNSPECIFIED. This takes care of
>         the situation where page reporting order is not specified
>         in the commandline.
> - Minor commit message changes.
> 
> Changes in v3:
> - Setting page_reporting_order's initial value to
> 	PAGE_REPORTING_ORDER_UNSPECIFIED moved to
> 	PATCH #5.
> 
> Changes in v4:
> - Move PAGE_REPORTING_ORDER_UNSPECIFIED's usage with
> 	page_reporting_order to patch #5.
> 
> Yuvraj Sakshith (5):
>   mm/page_reporting: add PAGE_REPORTING_ORDER_UNSPECIFIED
>   virtio_balloon: set unspecified page reporting order
>   hv_balloon: set unspecified page reporting order
>   mm/page_reporting: change PAGE_REPORTING_ORDER_UNSPECIFIED to -1
>   mm/page_reporting: change page_reporting_order to
>     PAGE_REPORTING_ORDER_UNSPECIFIED
> 
>  drivers/hv/hv_balloon.c         | 2 +-
>  drivers/virtio/virtio_balloon.c | 2 ++
>  include/linux/page_reporting.h  | 1 +
>  mm/page_reporting.c             | 7 ++++---
>  4 files changed, 8 insertions(+), 4 deletions(-)
> 
> -- 
> 2.34.1