[LTP] [PATCH 2/3] swapon03: Try to swapon() as many files until it fails

[Petr Vorel]

Previously tst_max_swapfiles() had fine tuning for a specific kernel
version which was fragile due various backports in enterprise kernels.

Let's try to create and use as many swap files until swapon() fails.
Then check for expected EPERM.

Suggested-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Petr Vorel <pvorel@suse.cz>
---
 testcases/kernel/syscalls/swapon/swapon03.c | 63 +++++++++++++--------
 1 file changed, 40 insertions(+), 23 deletions(-)

diff --git a/testcases/kernel/syscalls/swapon/swapon03.c b/testcases/kernel/syscalls/swapon/swapon03.c
index b89d853188..0027f874be 100644
--- a/testcases/kernel/syscalls/swapon/swapon03.c
+++ b/testcases/kernel/syscalls/swapon/swapon03.c
@@ -1,14 +1,18 @@
 // SPDX-License-Identifier: GPL-2.0-or-later
 /*
+ * Copyright (c) Linux Test Project, 2009-2025
  * Copyright (c) International Business Machines Corp., 2007
  * Created by <rsalveti@linux.vnet.ibm.com>
- *
  */
 
 /*\
- * This test case checks whether swapon(2) system call returns:
+ * Test checks whether :man2:`swapon` system call returns EPERM when the maximum
+ * number of swap files are already in use.
  *
- *  - EPERM when there are more than MAX_SWAPFILES already in use.
+ * NOTE: test does not try to calculate MAX_SWAPFILES from the internal
+ * kernel implementation (which is currently <23, 29> depending on kernel
+ * configuration). Instead test exptect that at least 15 swap files minus
+ * currently used swap can be created.
  */
 
 #include <stdio.h>
@@ -20,49 +24,54 @@
 #include "lapi/syscalls.h"
 #include "libswap.h"
 
+#define NUM_SWAP_FILES 15
 #define MNTPOINT	"mntpoint"
 #define TEST_FILE	MNTPOINT"/testswap"
 
-static int swapfiles;
+static int *swapfiles;
 
 static void setup_swap(void)
 {
 	pid_t pid;
-	int status;
-	int j, max_swapfiles, used_swapfiles;
+	int status, used_swapfiles, expected_swapfiles;
 	char filename[FILENAME_MAX];
 
 	SAFE_SETEUID(0);
 
-	/* Determine how many more files are to be created */
-	max_swapfiles = tst_max_swapfiles();
 	used_swapfiles = tst_count_swaps();
-	swapfiles = max_swapfiles - used_swapfiles;
-	if (swapfiles > max_swapfiles)
-		swapfiles = max_swapfiles;
+	expected_swapfiles = NUM_SWAP_FILES - used_swapfiles;
+
+	if (expected_swapfiles < 0) {
+		tst_brk(TCONF, "Warning: too many used swap files (%d)",
+			expected_swapfiles);
+	}
 
 	pid = SAFE_FORK();
 	if (pid == 0) {
-		/*create and turn on remaining swapfiles */
-		for (j = 0; j < swapfiles; j++) {
-
+		while (true) {
 			/* Create the swapfile */
-			snprintf(filename, sizeof(filename), "%s%02d", TEST_FILE, j + 2);
+			snprintf(filename, sizeof(filename), "%s%02d", TEST_FILE, *swapfiles);
 			MAKE_SMALL_SWAPFILE(filename);
 
-			/* turn on the swap file */
-			TST_EXP_PASS_SILENT(swapon(filename, 0));
+			/* Quit on a first swap file over max */
+			if (swapon(filename, 0) == -1)
+				break;
+			(*swapfiles)++;
 		}
 		exit(0);
 	} else {
 		waitpid(pid, &status, 0);
 	}
 
-	if (WEXITSTATUS(status))
+	if (WEXITSTATUS(status) || *swapfiles == 0)
 		tst_brk(TBROK, "Failed to setup swap files");
 
-	tst_res(TINFO, "Successfully created %d swap files", swapfiles);
-	MAKE_SMALL_SWAPFILE(TEST_FILE);
+	if (*swapfiles < expected_swapfiles) {
+		tst_res(TWARN, "Successfully created only %d swap files (>= %d expected)",
+			*swapfiles, expected_swapfiles);
+	} else {
+		tst_res(TINFO, "Successfully created %d swap files", *swapfiles);
+	}
 }
 
 /*
@@ -86,8 +95,8 @@ static void clean_swap(void)
 	int j;
 	char filename[FILENAME_MAX];
 
-	for (j = 0; j < swapfiles; j++) {
-		snprintf(filename, sizeof(filename), "%s%02d", TEST_FILE, j + 2);
+	for (j = 0; j < *swapfiles; j++) {
+		snprintf(filename, sizeof(filename), "%s%02d", TEST_FILE, j);
 		check_and_swapoff(filename);
 	}
 
@@ -105,12 +114,20 @@ static void setup(void)
 		tst_brk(TCONF, "swap not supported by kernel");
 
 	is_swap_supported(TEST_FILE);
+
+	swapfiles = SAFE_MMAP(NULL, sizeof(*swapfiles), PROT_READ | PROT_WRITE,
+			MAP_SHARED | MAP_ANONYMOUS, -1, 0);
+	*swapfiles = 0;
+
 	setup_swap();
 }
 
 static void cleanup(void)
 {
-	clean_swap();
+	if (swapfiles) {
+		clean_swap();
+		SAFE_MUNMAP(swapfiles, sizeof(*swapfiles));
+	}
 }
 
 static struct tst_test test = {
-- 
2.51.0