[PATCH v1 0/4] Allow order zero pages in page reporting

[PATCH v1 0/4] 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.

Yuvraj Sakshith (3):
  mm/page_reporting: Allow zero page_reporting_order
  hv_balloon: Change default page reporting order
  virtio_balloon: Set pr_dev.order to new default

 drivers/hv/hv_balloon.c         |  2 +-
 drivers/virtio/virtio_balloon.c | 14 ++++++++++++++
 mm/page_reporting.c             |  2 +-
 3 files changed, 16 insertions(+), 2 deletions(-)

-- 
2.34.1

[PATCH v1 1/4] page_reporting: add PAGE_REPORTING_DEFAULT_ORDER

[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_DEFAULT_ORDER and
check for it when the driver is being registered.

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..a7e3e30f2 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_DEFAULT_ORDER	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..9ad4fc3f8 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_DEFAULT_ORDER &&
+			prdev->order <= MAX_PAGE_ORDER)
 			page_reporting_order = prdev->order;
 		else
 			page_reporting_order = pageblock_order;
-- 
2.34.1

[PATCH v1 2/4] virtio_balloon: set default 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_DEFAULT ORDER fallback value.

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..0616c03b2 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_DEFAULT_ORDER;
+
 		/*
 		 * The default page reporting order is @pageblock_order, which
 		 * corresponds to 512MB in size on ARM64 when 64KB base page
-- 
2.34.1

[PATCH v1 3/4] hv_balloon: set default page reporting order

[Yuvraj Sakshith]

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

Reviewed-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..3d6bd9936 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_DEFAULT_ORDER;
 	ret = page_reporting_register(&dm_device.pr_dev_info);
 	if (ret < 0) {
 		dm_device.pr_dev_info.report = NULL;
-- 
2.34.1

[PATCH v1 4/4] page_reporting: change PAGE_REPORTING_DEFAULT_ORDER to -1

[Yuvraj Sakshith]

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

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 a7e3e30f2..3eb3e26d8 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_DEFAULT_ORDER	0
+#define PAGE_REPORTING_DEFAULT_ORDER	(-1)
 
 struct page_reporting_dev_info {
 	/* function that alters pages to make them "reported" */
-- 
2.34.1

Re: [PATCH v1 0/4] Allow order zero pages in page reporting

[David Hildenbrand (Arm)]

On 2/27/26 15:06, 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().
> 
> 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.
> 
> Yuvraj Sakshith (3):
>   mm/page_reporting: Allow zero page_reporting_order
>   hv_balloon: Change default page reporting order
>   virtio_balloon: Set pr_dev.order to new default

These look like old stats :)

-- 
Cheers,

David

Re: [PATCH v1 1/4] page_reporting: add PAGE_REPORTING_DEFAULT_ORDER

[David Hildenbrand (Arm)]

On 2/27/26 15:06, 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_DEFAULT_ORDER and
> check for it when the driver is being registered.

Patch subject: "mm/page_reporting:"

Might want to add "We'll make explicit use of this define in relevant
drivers next."

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


-- 
Cheers,

David

Re: [PATCH v1 2/4] virtio_balloon: set default page reporting order

[David Hildenbrand (Arm)]

On 2/27/26 15:06, Yuvraj Sakshith wrote:
> 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_DEFAULT ORDER fallback value.
> 
> Signed-off-by: Yuvraj Sakshith <yuvraj.sakshith@oss.qualcomm.com>

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

-- 
Cheers,

David

Re: [PATCH v1 4/4] page_reporting: change PAGE_REPORTING_DEFAULT_ORDER to -1

[David Hildenbrand (Arm)]

On 2/27/26 15:06, Yuvraj Sakshith wrote:
> PAGE_REPORTING_DEFAULT_ORDER 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_DEFAULT_ORDER to (-1),
> so that zero can be used as a valid order with which pages can
> be reported.
> 
> 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 a7e3e30f2..3eb3e26d8 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_DEFAULT_ORDER	0
> +#define PAGE_REPORTING_DEFAULT_ORDER	(-1)

No need for the ().

Wondering whether we now also want to do in this patch:


diff --git a/mm/page_reporting.c b/mm/page_reporting.c
index f0042d5743af..d432aadf9d07 100644
--- a/mm/page_reporting.c
+++ b/mm/page_reporting.c
@@ -11,8 +11,7 @@
 #include "page_reporting.h"
 #include "internal.h"

-/* Initialize to an unsupported value */
-unsigned int page_reporting_order = -1;
+unsigned int page_reporting_order = PAGE_REPORTING_DEFAULT_ORDER;

 static int page_order_update_notify(const char *val, const struct
kernel_param *kp)
 {
@@ -369,7 +368,7 @@ int page_reporting_register(struct
page_reporting_dev_info *prdev)
         * pageblock_order.
         */

-       if (page_reporting_order == -1) {
+       if (page_reporting_order == PAGE_REPORTING_DEFAULT_ORDER) {



(and wondering whether we should have called it
PAGE_REPORTING_USE_DEFAULT_ORDER to make it clearer that it is not an
actual order. Leaving that up to you :) )

-- 
Cheers,

David