[PATCH v4] mm/page_reporting: Add page_reporting_delay_ms module parameter

[pratmal@google.com]

From: Pratyush Mallick <pratmal@google.com>

Free page reporting currently hardcodes a 2-second interval between
reports. This rigid delay cannot accommodate diverse guest workloads.

This patch introduces a module parameter, page_reporting_delay_ms
(default: 2000), allowing users to tune the reporting rate:
 - Lower values enable aggressive memory reclamation by returning unused
   pages to the host immediately.
 - Higher values help batch pages during spiky allocation/free churn,
   reducing hypercalls and nested page fault overheads.

Setting the delay to 0 is safe and execution is strictly gated by:
 - reporting is only triggered by high-order page frees.
 - expensive hypercalls are bounded by a slot capacity watermark check
   before proceeding.

Signed-off-by: Pratyush Mallick <pratmal@google.com>
---
v4:
 - Added a helper function for schedule_delayed_work().
 - Updated the commit message.
 - v3: https://lore.kernel.org/linux-mm/amiPTPKfTOEpgetH@lucifer/T/#t
v3:
 - Converted page_reporting_delay_ms from a sysctl to a module parameter.
 - Dropped the max value cap (PAGE_REPORTING_DELAY_MS_MAX).
 - Documented page_reporting.page_reporting_delay_ms in Documentation/admin-guide/kernel-parameters.txt.
 - Updated code comments in mm/page_reporting.c to reflect dynamic parameterization.
 - v2: https://lore.kernel.org/linux-mm/3da27fde-25dc-4cb8-8e05-74cd26fc2f7c@kernel.org/T/#t
v2:
 - Documented page_reporting_delay_ms in Documentation/admin-guide/sysctl/vm.rst.
 - v1: https://lore.kernel.org/linux-mm/20260722192935.1646848-1-pratmal@google.com/T/#u
v1: Fixed feedback from RFC.
 - Added lower and upper cap to sysctl value.
 - Reverted the reordering on page_reporting_delay_ms.
 - Dropped the mod_delayed_work() change.
 - RFC: https://lore.kernel.org/linux-mm/20260714171456.2350037-1-pratmal@google.com/T/#u
 .../admin-guide/kernel-parameters.txt         |  6 +++++
 mm/page_reporting.c                           | 26 +++++++++++++------
 2 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index b5493a7f8f22..364c2dce8e70 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4810,6 +4810,12 @@ Kernel parameters
 			Adjust the minimal page reporting order. The page
 			reporting is disabled when it exceeds MAX_PAGE_ORDER.
 
+	page_reporting.page_reporting_delay_ms=
+			[KNL] Free page reporting delay in milliseconds
+			Format: <unsigned integer>
+			Adjust the delay in milliseconds between free page
+			reporting intervals. Default is 2000 (2 seconds).
+
 	panic=		[KNL] Kernel behaviour on panic: delay <timeout>
 			timeout > 0: seconds before rebooting
 			timeout = 0: wait forever
diff --git a/mm/page_reporting.c b/mm/page_reporting.c
index 942e84b6908a..c01aa3af80f3 100644
--- a/mm/page_reporting.c
+++ b/mm/page_reporting.c
@@ -47,7 +47,11 @@ MODULE_PARM_DESC(page_reporting_order, "Set page reporting order");
  */
 EXPORT_SYMBOL_GPL(page_reporting_order);
 
-#define PAGE_REPORTING_DELAY	(2 * HZ)
+static unsigned int page_reporting_delay_ms = 2 * MSEC_PER_SEC;
+module_param(page_reporting_delay_ms, uint, 0644);
+MODULE_PARM_DESC(page_reporting_delay_ms,
+		 "Set page reporting delay in milliseconds");
+
 static struct page_reporting_dev_info __rcu *pr_dev_info __read_mostly;
 
 enum {
@@ -56,6 +60,13 @@ enum {
 	PAGE_REPORTING_ACTIVE
 };
 
+/* schedule work for page reporting */
+static void page_reporting_schedule_work(struct page_reporting_dev_info *prdev)
+{
+	schedule_delayed_work(&prdev->work,
+			      msecs_to_jiffies(page_reporting_delay_ms));
+}
+
 /* request page reporting */
 static void
 __page_reporting_request(struct page_reporting_dev_info *prdev)
@@ -76,11 +87,10 @@ __page_reporting_request(struct page_reporting_dev_info *prdev)
 		return;
 
 	/*
-	 * Delay the start of work to allow a sizable queue to build. For
-	 * now we are limiting this to running no more than once every
-	 * couple of seconds.
+	 * Delay the start of work to allow a sizable queue to build.
+	 * We limit this based on page_reporting_delay_ms.
 	 */
-	schedule_delayed_work(&prdev->work, PAGE_REPORTING_DELAY);
+	page_reporting_schedule_work(prdev);
 }
 
 /* notify prdev of free page reporting request */
@@ -335,12 +345,12 @@ static void page_reporting_process(struct work_struct *work)
 err_out:
 	/*
 	 * If the state has reverted back to requested then there may be
-	 * additional pages to be processed. We will defer for 2s to allow
-	 * more pages to accumulate.
+	 * additional pages to be processed. We will defer by
+	 * page_reporting_delay_ms to allow more pages to accumulate.
 	 */
 	state = atomic_cmpxchg(&prdev->state, state, PAGE_REPORTING_IDLE);
 	if (state == PAGE_REPORTING_REQUESTED)
-		schedule_delayed_work(&prdev->work, PAGE_REPORTING_DELAY);
+		page_reporting_schedule_work(prdev);
 }
 
 static DEFINE_MUTEX(page_reporting_mutex);
-- 
2.55.0.508.g3f0d502094-goog