[RFC PATCH 50/57] selftests/mm: attribute collapses by candidate event alone

[Kiryl Shutsemau <kirill@shutemov.name>]

From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>

The check counts collapses of its window by matching isolate events
against the source PFNs it recorded beforehand.  That tracepoint went with
the mechanism that emitted it, so nothing would match.

Count the engine's per-candidate events instead: an install that
succeeded, at the window's address and order, is one collapse of that
window.

Preparing the window still checks that the sources are present, which is
the other thing those PFN lookups were doing.  The recorded PFNs
themselves are no longer needed, so the array goes.

Assisted-by: Claude-Code:claude-opus-5
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
---
 .../selftests/mm/khugepaged_sync_check.c      | 55 +++++++++----------
 1 file changed, 26 insertions(+), 29 deletions(-)

diff --git a/tools/testing/selftests/mm/khugepaged_sync_check.c b/tools/testing/selftests/mm/khugepaged_sync_check.c
index 4c37b697d3dd..2a0c247aeec5 100644
--- a/tools/testing/selftests/mm/khugepaged_sync_check.c
+++ b/tools/testing/selftests/mm/khugepaged_sync_check.c
@@ -7,10 +7,9 @@
  * advancing by two is a completion barrier for one full pass that
  * started after setup (khugepaged_full_pass()). Verify the pair gives
  * deterministic, attributable results: one barrier step over one
- * prepared window produces exactly one collapse attempt on that
- * window's source pages (mm_collapse_huge_page_isolate events filtered
- * by source PFN and order) and the window is collapsed
- * afterwards, repeatably.
+ * prepared window produces exactly one collapse of that window --
+ * mm_collapse_candidate install events at its address and order -- and
+ * the window is collapsed afterwards, repeatably.
  *
  * scan_sleep_millisecs is set to 60s to prove the wake path: without
  * the wake, one barrier step would sleep multiples of that and blow
@@ -51,11 +50,10 @@ static void trace_events_off(void)
 }
 
 /*
- * Count collapse attempts attributable to our window: isolate events whose
- * scan_pfn is one of the window's source PFNs, reported once per attempt.
+ * Count the collapses attributable to our window: per-candidate install
+ * events at the window's address and order, one per collapse.
  */
-static int count_attributed(unsigned long *pfns, int nr_pfns,
-			    unsigned int order)
+static int count_attributed(unsigned long addr, unsigned int order)
 {
 	char line[1024];
 	int count = 0;
@@ -66,25 +64,25 @@ static int count_attributed(unsigned long *pfns, int nr_pfns,
 		ksft_exit_fail_msg("Cannot open trace buffer\n");
 
 	while (fgets(line, sizeof(line), fp)) {
+		char *s;
 		unsigned long val;
 		unsigned int ord;
-		char *s, *o;
-		int i;
+		char *o;
 
-		s = strstr(line, "mm_collapse_huge_page_isolate:");
-		if (!s)
-			continue;
-		if (sscanf(s, "mm_collapse_huge_page_isolate: scan_pfn=0x%lx",
-			   &val) != 1)
-			continue;
-		o = strstr(s, "order=");
-		if (!o || sscanf(o, "order=%u", &ord) != 1 || ord != order)
-			continue;
-		for (i = 0; i < nr_pfns; i++) {
-			if (val == pfns[i]) {
-				count++;
-				break;
-			}
+		s = strstr(line, "mm_collapse_candidate:");
+		if (s) {
+			if (!strstr(s, "pass=install") ||
+			    !strstr(s, "result=succeeded"))
+				continue;
+			o = strstr(s, "addr=");
+			if (!o || sscanf(o, "addr=0x%lx", &val) != 1 ||
+			    val != addr)
+				continue;
+			o = strstr(s, "order=");
+			if (!o || sscanf(o, "order=%u", &ord) != 1 ||
+			    ord != order)
+				continue;
+			count++;
 		}
 	}
 	fclose(fp);
@@ -95,7 +93,6 @@ static void one_step(int iteration)
 {
 	const size_t window = getpagesize() << TARGET_ORDER;
 	const int nr_pages = 1 << TARGET_ORDER;
-	unsigned long pfns[1 << TARGET_ORDER];
 	bool collapsed, passed;
 	int attributed;
 	char *p;
@@ -106,11 +103,11 @@ static void one_step(int iteration)
 	if (p != BASE_ADDR)
 		ksft_exit_fail_perror("mmap() window");
 
-	/* Prepare one window; record its source PFNs. */
+	/* Prepare one window, and check the sources really are present. */
 	for (i = 0; i < nr_pages; i++) {
 		p[i * getpagesize()] = i + 1;
-		pfns[i] = pagemap_get_pfn(pagemap_fd, p + i * getpagesize());
-		if (pfns[i] == -1UL)
+		if (pagemap_get_pfn(pagemap_fd,
+				    p + i * getpagesize()) == -1UL)
 			ksft_exit_fail_msg("Source page not present\n");
 	}
 
@@ -133,7 +130,7 @@ static void one_step(int iteration)
 
 	collapsed = is_range_backed_by_folio_orders(p, window, TARGET_ORDER,
 						    pagemap_fd, kpageflags_fd);
-	attributed = count_attributed(pfns, nr_pages, TARGET_ORDER);
+	attributed = count_attributed((unsigned long)p, TARGET_ORDER);
 
 	ksft_test_result(collapsed && attributed == 1,
 			 "step %d: window collapsed, %d attributed result(s)\n",
-- 
2.54.0