[RFC PATCH 5/5] mm/damon/paddr: add time budget to migration page walk

[Ravi Jonnalagadda <ravis.opensrc@gmail.com>]

On populated physical address ranges the pageblock skip optimization
alone is insufficient — most pageblocks contain at least one allocated
page, so the walk still iterates millions of PFNs.

Add a 100ms wall-clock time budget to damon_pa_migrate().  Once the
deadline is reached, the walk breaks out and migrates whatever folios
have been collected so far.

The time check is amortized by only calling ktime_get() every 4096
pages (~16MB of address space), adding negligible overhead to the
fast path.

Signed-off-by: Ravi Jonnalagadda <ravis.opensrc@gmail.com>
---
 mm/damon/paddr.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/mm/damon/paddr.c b/mm/damon/paddr.c
index e844c990987b9..a2565287bc10f 100644
--- a/mm/damon/paddr.c
+++ b/mm/damon/paddr.c
@@ -14,6 +14,7 @@
 #include <linux/swap.h>
 #include <linux/memory-tiers.h>
 #include <linux/mm_inline.h>
+#include <linux/ktime.h>
 
 #include "../internal.h"
 #include "ops-common.h"
@@ -254,6 +255,14 @@ static unsigned long damon_pa_deactivate_pages(struct damon_region *r,
 	return damon_pa_de_activate(r, addr_unit, s, false, sz_filter_passed);
 }
 
+/* Maximum wall-clock time to spend in a single migration walk (ns) */
+#define DAMON_PA_MIGRATE_BUDGET_NS	(100 * NSEC_PER_MSEC)
+
+/* Check the time budget every 4096 pages (~16MB) to amortize ktime_get(). */
+#define DAMON_PA_MIGRATE_TIME_CHECK_PAGES	4096
+#define DAMON_PA_MIGRATE_TIME_CHECK_MASK	\
+	(DAMON_PA_MIGRATE_TIME_CHECK_PAGES - 1)
+
 static unsigned long damon_pa_migrate(struct damon_region *r,
 		unsigned long addr_unit, struct damos *s,
 		unsigned long *sz_filter_passed)
@@ -262,6 +271,7 @@ static unsigned long damon_pa_migrate(struct damon_region *r,
 	LIST_HEAD(folio_list);
 	struct folio *folio = NULL;
 	unsigned long pfn;
+	ktime_t deadline = ktime_add_ns(ktime_get(), DAMON_PA_MIGRATE_BUDGET_NS);
 
 	addr = damon_pa_phys_addr(r->ar.start, addr_unit);
 	end = damon_pa_phys_addr(r->ar.end, addr_unit);
@@ -283,6 +293,11 @@ static unsigned long damon_pa_migrate(struct damon_region *r,
 			}
 		}
 
+		/* Time budget: keep kdamond responsive on long migration walks. */
+		if (!(pfn & DAMON_PA_MIGRATE_TIME_CHECK_MASK) &&
+		    ktime_after(ktime_get(), deadline))
+			break;
+
 		folio = damon_get_folio(pfn);
 		if (damon_pa_invalid_damos_folio(folio, s)) {
 			addr += PAGE_SIZE;
-- 
2.43.0