selftests/mm: fix read_file() return value check

read_file() returns 0 on open/read failures and never returns negative
values.  Existing < 0 error checks never trigger, so read failures are
silently ignored.  Check for zero return to detect read_file() failures.

Also fix misleading error message in get_finfo().  The error string
incorrectly references read_num when reading uevent files.

Link: https://lore.kernel.org/20260807013555.36525-1-hongfu.li@linux.dev
Fixes: e0c13f9761 ("khugepaged: add self test")
Signed-off-by: Hongfu Li <lihongfu@kylinos.cn>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/mm/khugepaged.c

@@ -141,8 +141,8 @@ static void get_finfo(const char *dir)
 		     major(path_stat.st_dev), minor(path_stat.st_dev))
 	    >= sizeof(path))
 		ksft_exit_fail_msg("%s: Pathname is too long\n", __func__);
-	if (read_file(path, buf, sizeof(buf)) < 0)
-		ksft_exit_fail_perror("read_file(read_num)");
+	if (!read_file(path, buf, sizeof(buf)))
+		ksft_exit_fail_perror("read_file(uevent)");
 	if (strstr(buf, "DEVTYPE=disk")) {
 		/* Found it */
 		if (snprintf(finfo.dev_queue_read_ahead_path,

tools/testing/selftests/mm/vm_util.c

@@ -942,7 +942,7 @@ unsigned long read_num(const char *path)
 {
 	char buf[21];
 
-	if (read_file(path, buf, sizeof(buf)) < 0)
+	if (!read_file(path, buf, sizeof(buf)))
 		ksft_exit_fail_perror("read_file()");
 
 	return strtoul(buf, NULL, 10);