[PATCH v2 0/4] Allow order zero pages in page reporting

[PATCH v2 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.

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.

Yuvraj Sakshith (4):
  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

 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 v2 1/4] 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            | 14 +++++---------
 2 files changed, 6 insertions(+), 9 deletions(-)

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..51cd88faf 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)
 {
@@ -25,12 +25,7 @@ static int page_order_update_notify(const char *val, const struct kernel_param *
 
 static const struct kernel_param_ops page_reporting_param_ops = {
 	.set = &page_order_update_notify,
-	/*
-	 * For the get op, use param_get_int instead of param_get_uint.
-	 * This is to make sure that when unset the initialized value of
-	 * -1 is shown correctly
-	 */
-	.get = &param_get_int,
+	.get = &param_get_uint,
 };
 
 module_param_cb(page_reporting_order, &page_reporting_param_ops,
@@ -369,8 +364,9 @@ int page_reporting_register(struct page_reporting_dev_info *prdev)
 	 * pageblock_order.
 	 */
 
-	if (page_reporting_order == -1) {
-		if (prdev->order > 0 && prdev->order <= MAX_PAGE_ORDER)
+	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;
 		else
 			page_reporting_order = pageblock_order;
-- 
2.34.1

Re: [PATCH v2 1/4] mm/page_reporting: add PAGE_REPORTING_ORDER_UNSPECIFIED

[David Hildenbrand (Arm)]

On 3/2/26 12:17, 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            | 14 +++++---------
>  2 files changed, 6 insertions(+), 9 deletions(-)
> 
> 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..51cd88faf 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)
>  {
> @@ -25,12 +25,7 @@ static int page_order_update_notify(const char *val, const struct kernel_param *
>  
>  static const struct kernel_param_ops page_reporting_param_ops = {
>  	.set = &page_order_update_notify,
> -	/*
> -	 * For the get op, use param_get_int instead of param_get_uint.
> -	 * This is to make sure that when unset the initialized value of
> -	 * -1 is shown correctly
> -	 */
> -	.get = &param_get_int,
> +	.get = &param_get_uint,
>  };
>  
>  module_param_cb(page_reporting_order, &page_reporting_param_ops,
> @@ -369,8 +364,9 @@ int page_reporting_register(struct page_reporting_dev_info *prdev)
>  	 * pageblock_order.
>  	 */
>  
> -	if (page_reporting_order == -1) {
> -		if (prdev->order > 0 && prdev->order <= MAX_PAGE_ORDER)
> +	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;
>  		else
>  			page_reporting_order = pageblock_order;

I think the change to page_reporting_order (and param_get_int) should
come after patch #4.

Otherwise, you temporarily change the semantics of
page_reporting_param_ops() etc.

So you should perform the page_reporting_order changes either in patch
#4 or in a new patch #5.

Apart from that LGTM.

-- 
Cheers,

David

[PATCH v2 2/4] 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.

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 v2 3/4] 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.

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 v2 4/4] 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.

Signed-off-by: Yuvraj Sakshith <yuvraj.sakshith@oss.qualcomm.com>
---
 include/linux/page_reporting.h | 2 +-
 mm/page_reporting.c            | 7 ++++++-
 2 files changed, 7 insertions(+), 2 deletions(-)

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" */
diff --git a/mm/page_reporting.c b/mm/page_reporting.c
index 51cd88faf..21c11b75e 100644
--- a/mm/page_reporting.c
+++ b/mm/page_reporting.c
@@ -25,7 +25,12 @@ static int page_order_update_notify(const char *val, const struct kernel_param *
 
 static const struct kernel_param_ops page_reporting_param_ops = {
 	.set = &page_order_update_notify,
-	.get = &param_get_uint,
+	/*
+	 * For the get op, use param_get_int instead of param_get_uint.
+	 * This is to make sure that when unset the initialized value of
+	 * -1 is shown correctly
+	 */
+	.get = &param_get_int,
 };
 
 module_param_cb(page_reporting_order, &page_reporting_param_ops,
-- 
2.34.1