[PATCH 0/3] Allow order zero pages in page reporting

[PATCH 0/3] 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().

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 1/3] mm/page_reporting: Allow zero page_reporting_order

[Yuvraj Sakshith]

Some drivers might require page sized chunks to be
reported. This patch allows registering a driver with
order as  zero.

Example use case: virtio-balloon driver running on a
guest with very small memory. After some time has passed,
the guest might not be able to find a chunk of 8KB.

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

diff --git a/mm/page_reporting.c b/mm/page_reporting.c
index e4c428e61..fd7c5f0de 100644
--- a/mm/page_reporting.c
+++ b/mm/page_reporting.c
@@ -370,7 +370,7 @@ 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 >= 0 && prdev->order <= MAX_PAGE_ORDER)
 			page_reporting_order = prdev->order;
 		else
 			page_reporting_order = pageblock_order;
-- 
2.34.1

[PATCH 2/3] hv_balloon: Change default page reporting order

[Yuvraj Sakshith]

page_reporting_order used to fall back to default
value (passed as parameter or MAX_PAGE_ORDER) if
the driver wishes to not provide it.

The way the driver used to do this was by passing
the order as zero.

Now that zero is a valid order that can be passed by
a driver to page reporting, we use -1 to signal
default value to be used.

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..e33d6e3b2 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 = -1;
 	ret = page_reporting_register(&dm_device.pr_dev_info);
 	if (ret < 0) {
 		dm_device.pr_dev_info.report = NULL;
-- 
2.34.1

[PATCH 3/3] virtio_balloon: Set pr_dev.order to new default

[Yuvraj Sakshith]

Drivers registering with page reporting used zero
as a way to signal page_reporting_order to be set
as a default value (either passed as a param or
MAX_PAGE_ORDER).

Since page_reporting_order can now have zero as
valid order, default fallback value send by drivers
to page reporting is now -1.

Signed-off-by: Yuvraj Sakshith <yuvraj.sakshith@oss.qualcomm.com>
---
 drivers/virtio/virtio_balloon.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 74fe59f5a..3cc3dc28a 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -1044,6 +1044,20 @@ static int virtballoon_probe(struct virtio_device *vdev)
 			goto out_unregister_oom;
 		}
 
+		/*
+		 * page_reporting_register() takes the order either
+		 * from the driver or the commandline. If neither
+		 * are provided, it falls back to MAX_PAGE_ORDER.
+		 *
+		 * Order given by the driver is required to be in the
+		 * range [0, MAX_PAGE_ORDER].
+		 *
+		 * One way for the driver to not provide any order
+		 * is by setting it to -1.
+		 */
+
+		vb->pr_dev_info.order = -1;
+
 		/*
 		 * The default page reporting order is @pageblock_order, which
 		 * corresponds to 512MB in size on ARM64 when 64KB base page
-- 
2.34.1

Re: [PATCH 2/3] hv_balloon: Change default page reporting order

[David Hildenbrand (Arm)]

On 2/26/26 08:01, Yuvraj Sakshith wrote:
> page_reporting_order used to fall back to default
> value (passed as parameter or MAX_PAGE_ORDER) if
> the driver wishes to not provide it.
> 
> The way the driver used to do this was by passing
> the order as zero.
> 
> Now that zero is a valid order that can be passed by
> a driver to page reporting, we use -1 to signal
> default value to be used.
> 
> 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..e33d6e3b2 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 = -1;
>  	ret = page_reporting_register(&dm_device.pr_dev_info);
>  	if (ret < 0) {
>  		dm_device.pr_dev_info.report = NULL;

Logically, that patch must come before #1. And the patch description
should be rephrased to clarify that we want to change that behavior.

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

-- 
Cheers,

David

Re: [PATCH 3/3] virtio_balloon: Set pr_dev.order to new default

[David Hildenbrand (Arm)]

On 2/26/26 08:01, Yuvraj Sakshith wrote:
> Drivers registering with page reporting used zero
> as a way to signal page_reporting_order to be set
> as a default value (either passed as a param or
> MAX_PAGE_ORDER).
> 
> Since page_reporting_order can now have zero as
> valid order, default fallback value send by drivers
> to page reporting is now -1.
> 
> Signed-off-by: Yuvraj Sakshith <yuvraj.sakshith@oss.qualcomm.com>
> ---
>  drivers/virtio/virtio_balloon.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 74fe59f5a..3cc3dc28a 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -1044,6 +1044,20 @@ static int virtballoon_probe(struct virtio_device *vdev)
>  			goto out_unregister_oom;
>  		}
>  
> +		/*
> +		 * page_reporting_register() takes the order either
> +		 * from the driver or the commandline. If neither
> +		 * are provided, it falls back to MAX_PAGE_ORDER.
> +		 *
> +		 * Order given by the driver is required to be in the
> +		 * range [0, MAX_PAGE_ORDER].
> +		 *
> +		 * One way for the driver to not provide any order
> +		 * is by setting it to -1.
> +		 */
> +
> +		vb->pr_dev_info.order = -1;

That overly-long comment indicates that we can do better.


What about the following:

Patch #1: Introduce PAGE_REPORTING_DEFAULT_ORDER

diff --git a/include/linux/page_reporting.h b/include/linux/page_reporting.h
index fe648dfa3a7c..3e21bfbb49a4 100644
--- a/include/linux/page_reporting.h
+++ b/include/linux/page_reporting.h
@@ -8,6 +8,9 @@
 /* This value should always be a power of 2, see page_reporting_cycle() */
 #define PAGE_REPORTING_CAPACITY                32
 
+/* Specifying this value as minimal reporting order selects the default. */
+#define PAGE_REPORTING_DEFAULT_ORDER   0
+
 struct page_reporting_dev_info {
        /* function that alters pages to make them "reported" */
        int (*report)(struct page_reporting_dev_info *prdev,
diff --git a/mm/page_reporting.c b/mm/page_reporting.c
index f0042d5743af..d5191cb6b31c 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;


Patch #2: Use the define in hyperv

Patch #3: Use the define in virtio-balloon

Patch #4: Change PAGE_REPORTING_DEFAULT_ORDER to "MAX_PAGE_ORDER + 1" or sth. like that.


Then I think you can just drop the comment in virtballoon_probe() completely.

-- 
Cheers,

David