[PATCH v3 18/18] selftests/mm: zap whole PTE tables in the khugepaged race harness

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@xxxxxxxxxx>

The harness's MADV_DONTNEED thread zaps 1 to 32 pages at a time, never a
whole PMD-aligned area.  Empty-table reclaim (CONFIG_PT_RECLAIM) only
engages when a zap spans a full table, so no soak ever ran it against a
collapse -- fuzzing had to find that class instead: the page table
vanishing between the engine's park and install passes.

Make the thread zap a whole PMD-aligned area once every 64 iterations, and
keep the fine-grained zaps as the common case.

Assisted-by: Claude-Code:claude-opus-5
Tested-by: Muhammad Usama Anjum <usama.anjum@xxxxxxx>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
---
 tools/testing/selftests/mm/khugepaged_race.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/mm/khugepaged_race.c b/tools/testing/selftests/mm/khugepaged_race.c
index bde36af03e49..3601c030d43e 100644
--- a/tools/testing/selftests/mm/khugepaged_race.c
+++ b/tools/testing/selftests/mm/khugepaged_race.c
@@ -141,8 +141,24 @@ static void *dontneed_fn(void *arg)
 		unsigned long page_idx = rand_page(&seed);
 		unsigned long nr = 1UL << (rand_r(&seed) % 6);	/* 1..32 pages */
 
-		madvise(region + page_idx * page_size,
-			room_from(page_idx, nr) * page_size, MADV_DONTNEED);
+		/*
+		 * Once in a while zap a whole PMD-aligned area: only a zap
+		 * spanning the full table triggers the empty-table reclaim
+		 * (CONFIG_PT_RECLAIM), which can free the table under a
+		 * collapse that is midway through it.  Sub-table zaps never
+		 * reach that path.
+		 */
+		if (!(rand_r(&seed) % 64)) {
+			unsigned long area = page_idx /
+					(hpage_pmd_size / page_size);
+
+			madvise(region + area * hpage_pmd_size,
+				hpage_pmd_size, MADV_DONTNEED);
+		} else {
+			madvise(region + page_idx * page_size,
+				room_from(page_idx, nr) * page_size,
+				MADV_DONTNEED);
+		}
 		usleep(rand_r(&seed) % 500);
 	}
 	return NULL;
-- 
2.54.0