[LTP] [PATCH v5] swapon03: Try to swapon() as many files until it fails

Petr Vorel
2025-12-22T09:45:02Z

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.

It was required to increase cmd_buffer size to avoid directive output
may be truncated warning.

Suggested-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Petr Vorel <pvorel@suse.cz>
---
v4->v5:
* Use tst_brk(TFAIL | TERRNO) to avoid infinite loop (Li).
* Minor fix in doc (missing bracket).

Link to v4:
https://lore.kernel.org/ltp/20251219094219.151887-1-pvorel@suse.cz/

Note: I'll plan to do followup fixes in beginning of January (nicely
summarized by Li [1] [2]):

is_swapfile_supported():
- swap syscalls present (/proc/swaps exists)
- filesystem allows creating files (has enough space)
- can mkswap (or whatever MAKE_SMALL_SWAPFILE does) successfully
- possibly sanity-check that swapfiles are supported on this FS

is_swapon_supported():
- call is_swapfile_supported()
- create a temporary swapfile
- swapon() / swapoff() it
- clean up

[1] https://lore.kernel.org/ltp/CAEemH2f2nhCBzxw-5u5qGnDS9BcogD-KWOd+mrOoLvmJ0XPR9w@mail.gmail.com/
[2] https://lore.kernel.org/ltp/CAEemH2dts8FsEAM7gfKQjSv3ohkVehy9fXSf09_bqLfoDnUf1g@mail.gmail.com/

testcases/kernel/syscalls/swapon/swapon03.c | 49 ++++++++++++---------
1 file changed, 27 insertions(+), 22 deletions(-)

diff --git a/testcases/kernel/syscalls/swapon/swapon03.c b/testcases/kernel/syscalls/swapon/swapon03.c
index c014a48912..53c5750de4 100644
--- a/testcases/kernel/syscalls/swapon/swapon03.c
+++ b/testcases/kernel/syscalls/swapon/swapon03.c
@@ -6,9 +6,12 @@
*/

/*\
- * 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, instead make sure at least 15 swaps were created
+ * before the maximum of swaps was reached.
*/

#include <stdio.h>
@@ -20,6 +23,13 @@
#include "lapi/syscalls.h"
#include "libswap.h"

+/*
+ * MAX_SWAPFILES from the internal kernel implementation is currently <23, 29>,
+ * depending on kernel configuration (see man swapon(2)). Chose small enough
+ * value for future changes.
+ */
+#define MIN_SWAP_FILES 15
+
#define MNTPOINT "mntpoint"
#define TEST_FILE MNTPOINT"/testswap"

@@ -27,31 +37,28 @@ static int swapfiles;

static int setup_swap(void)
{
- int j, max_swapfiles, used_swapfiles;
+ int used_swapfiles, min_swapfiles;
char filename[FILENAME_MAX];

- /* 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;
-
- /*create and turn on remaining swapfiles */
- for (j = 0; j < swapfiles; j++) {
+ min_swapfiles = MIN_SWAP_FILES - used_swapfiles;

+ while (true) {
/* Create the swapfile */
- snprintf(filename, sizeof(filename), "%s%02d", TEST_FILE, j + 2);
- SAFE_MAKE_SMALL_SWAPFILE(filename);
+ snprintf(filename, sizeof(filename), "%s%02d", TEST_FILE, swapfiles);
+ MAKE_SMALL_SWAPFILE(filename);
+
+ /* Quit on a first swap file over max, check for EPERM */
+ if (swapon(filename, 0) == -1) {
+ if (errno == EPERM && swapfiles > min_swapfiles)
+ break;

- /* turn on the swap file */
- TST_EXP_PASS_SILENT(swapon(filename, 0));
- if (!TST_PASS)
- tst_brk(TFAIL, "Failed to setup swap files");
+ tst_brk(TFAIL | TERRNO, "swapon(%s, 0)", filename);
+ }
+ swapfiles++;
}

tst_res(TINFO, "Successfully created %d swap files", swapfiles);
- MAKE_SMALL_SWAPFILE(TEST_FILE);

return 0;
}
@@ -61,7 +68,7 @@ static int setup_swap(void)
*/
static int check_and_swapoff(const char *filename)
{
- char cmd_buffer[256];
+ char cmd_buffer[FILENAME_MAX+28];
int rc = -1;

snprintf(cmd_buffer, sizeof(cmd_buffer), "grep -q '%s.*file' /proc/swaps", filename);
@@ -83,11 +90,9 @@ static void clean_swap(void)
char filename[FILENAME_MAX];

for (j = 0; j < swapfiles; j++) {
- snprintf(filename, sizeof(filename), "%s%02d", TEST_FILE, j + 2);
+ snprintf(filename, sizeof(filename), "%s%02d", TEST_FILE, j);
check_and_swapoff(filename);
}
-
- check_and_swapoff(TEST_FILE);
}

static void verify_swapon(void)
--
2.51.0