mm/page_reporting: add page_reporting_delay_ms module parameter

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.

Link: https://lore.kernel.org/[email protected]
Signed-off-by: Pratyush Mallick <[email protected]>
Reviewed-by: SJ Park <[email protected]>
Acked-by: Lorenzo Stoakes (ARM) <[email protected]>
Acked-by: David Hildenbrand (Arm) <[email protected]>
Cc: Anshuman Khandual <[email protected]>
Cc: Brendan Jackman <[email protected]>
Cc: Greg Thelen <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Liam R. Howlett <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: SeongJae Park <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Zi Yan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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

mm/page_reporting.c

@@ -48,7 +48,11 @@ 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 {
@@ -57,6 +61,13 @@ enum {
 	PAGE_REPORTING_ACTIVE
 };
 
+/* schedule work for page reporting */
+static void page_reporting_schedule_work(struct page_reporting_dev_info *prdev)
+{
+	queue_delayed_work(system_freezable_wq, &prdev->work,
+			   msecs_to_jiffies(page_reporting_delay_ms));
+}
+
 /* request page reporting */
 static void
 __page_reporting_request(struct page_reporting_dev_info *prdev)
@@ -77,12 +88,10 @@ __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.
 	 */
-	queue_delayed_work(system_freezable_wq, &prdev->work,
-			   PAGE_REPORTING_DELAY);
+	page_reporting_schedule_work(prdev);
 }
 
 /* notify prdev of free page reporting request */
@@ -337,13 +346,12 @@ 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)
-		queue_delayed_work(system_freezable_wq, &prdev->work,
-				   PAGE_REPORTING_DELAY);
+		page_reporting_schedule_work(prdev);
 }
 
 static DEFINE_MUTEX(page_reporting_mutex);