[PATCH RESEND v1 5/5] selftests/mm: retry migration failures for the full runtime

[Muhammad Usama Anjum <usama.anjum@arm.com>]

move_pages() migration is best effort and may temporarily leave a page
unmigrated when another task faults on the folio during unmapping.  The
migration test currently tolerates 100 consecutive partial failures,
but a busy machine can exhaust those retries in a fraction of a second
and fail long before the intended 20-second runtime expires.

This is especially visible in the shared-anon cases, where every worker
process continuously accesses the same mapping.  The retry count then
measures syscall rate rather than whether the test can exercise a
successful migration entry.

Retry positive move_pages() results for the full runtime and record the
per-page status.  Pass if at least one migration succeeds, and report the
last status before failing if no migration makes progress.  Continue to
fail immediately for syscall-level errors.

Signed-off-by: Muhammad Usama Anjum <usama.anjum@arm.com>
---
 tools/testing/selftests/mm/migration.c | 34 +++++++++++++++++---------
 1 file changed, 22 insertions(+), 12 deletions(-)

diff --git a/tools/testing/selftests/mm/migration.c b/tools/testing/selftests/mm/migration.c
index 29f7492453d43..19b4cef65f345 100644
--- a/tools/testing/selftests/mm/migration.c
+++ b/tools/testing/selftests/mm/migration.c
@@ -7,7 +7,7 @@
 #include "kselftest_harness.h"
 #include "hugepage_settings.h"
 
-#include <strings.h>
+#include <string.h>
 #include <pthread.h>
 #include <numa.h>
 #include <numaif.h>
@@ -20,7 +20,6 @@
 
 #define TWOMEG		(2<<20)
 #define RUNTIME		(20)
-#define MAX_RETRIES	100
 #define ALIGN(x, a)	(((x) + (a - 1)) & (~((a) - 1)))
 
 HUGETLB_SETUP_DEFAULT_PAGES(1)
@@ -110,7 +109,8 @@ int migrate(uint64_t *ptr, int n1, int n2)
 	int ret, tmp;
 	int status = 0;
 	struct timespec ts1, ts2;
-	int failures = 0;
+	int last_status = 0;
+	unsigned long successes = 0;
 
 	if (clock_gettime(CLOCK_MONOTONIC, &ts1))
 		return -1;
@@ -119,23 +119,33 @@ int migrate(uint64_t *ptr, int n1, int n2)
 		if (clock_gettime(CLOCK_MONOTONIC, &ts2))
 			return -1;
 
-		if (ts2.tv_sec - ts1.tv_sec >= RUNTIME)
-			return 0;
+		if (ts2.tv_sec - ts1.tv_sec >= RUNTIME) {
+			if (successes)
+				return 0;
 
+			if (last_status < 0)
+				printf("No page migration succeeded: %s (%d)\n",
+				       strerror(-last_status), last_status);
+			else
+				printf("No page migration succeeded: status %d\n",
+				       last_status);
+			return -2;
+		}
+
+		status = 0;
 		ret = move_pages(0, 1, (void **) &ptr, &n2, &status,
 				MPOL_MF_MOVE_ALL);
 		if (ret) {
 			if (ret > 0) {
-				/* Migration is best effort; try again */
-				if (++failures < MAX_RETRIES)
-					continue;
-				printf("Didn't migrate %d pages\n", ret);
-			}
-			else
+				/* Migration is best effort; try again. */
+				last_status = status;
+				continue;
+			} else {
 				perror("Couldn't migrate pages");
+			}
 			return -2;
 		}
-		failures = 0;
+		successes++;
 		tmp = n2;
 		n2 = n1;
 		n1 = tmp;
-- 
2.47.3