Re: [PATCH v2 2/2] selftests/cgroup: add test for zswap incompressible pages

[Nhat Pham <nphamcs@gmail.com>]

On Thu, Feb 5, 2026 at 11:22 PM Jiayuan Chen <jiayuan.chen@linux.dev> wrote:
>
> From: Jiayuan Chen <jiayuan.chen@shopee.com>
>
> Add test_zswap_incompressible() to verify that the zswap_incomp memcg
> stat correctly tracks incompressible pages.
>
> The test allocates memory filled with random data from /dev/urandom,
> which cannot be effectively compressed by zswap. When this data is
> swapped out to zswap, it should be stored as-is and tracked by the
> zswap_incomp counter.
>
> The test verifies that:
> 1. Pages are swapped out to zswap (zswpout increases)
> 2. Incompressible pages are tracked (zswap_incomp increases)
>
> test:
> dd if=/dev/zero of=/swapfile bs=1M count=2048
> chmod 600 /swapfile
> mkswap /swapfile
> swapon /swapfile
> echo Y > /sys/module/zswap/parameters/enabled
>
> ./test_zswap
>  TAP version 13
>  1..8
>  ok 1 test_zswap_usage
>  ok 2 test_swapin_nozswap
>  ok 3 test_zswapin
>  ok 4 test_zswap_writeback_enabled
>  ok 5 test_zswap_writeback_disabled
>  ok 6 test_no_kmem_bypass
>  ok 7 test_no_invasive_cgroup_shrink
>  ok 8 test_zswap_incompressible
>  Totals: pass:8 fail:0 xfail:0 xpass:0 skip:0 error:0
>
> Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
> ---
>  tools/testing/selftests/cgroup/test_zswap.c | 96 +++++++++++++++++++++
>  1 file changed, 96 insertions(+)
>
> diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c
> index 64ebc3f3f203..8cb8a131357d 100644
> --- a/tools/testing/selftests/cgroup/test_zswap.c
> +++ b/tools/testing/selftests/cgroup/test_zswap.c
> @@ -5,6 +5,7 @@
>  #include <unistd.h>
>  #include <stdio.h>
>  #include <signal.h>
> +#include <fcntl.h>
>  #include <sys/sysinfo.h>
>  #include <string.h>
>  #include <sys/wait.h>
> @@ -574,6 +575,100 @@ static int test_no_kmem_bypass(const char *root)
>         return ret;
>  }
>
> +static int allocate_random_and_wait(const char *cgroup, void *arg)
> +{
> +       size_t size = (size_t)arg;
> +       char *mem;
> +       int fd;
> +       ssize_t n;
> +
> +       mem = malloc(size);
> +       if (!mem)
> +               return -1;
> +
> +       /* Fill with random data from /dev/urandom - incompressible */
> +       fd = open("/dev/urandom", O_RDONLY);
> +       if (fd < 0) {
> +               free(mem);
> +               return -1;
> +       }
> +
> +       for (size_t i = 0; i < size; ) {
> +               n = read(fd, mem + i, size - i);
> +               if (n <= 0)
> +                       break;
> +               i += n;
> +       }
> +       close(fd);
> +
> +       /* Touch all pages to ensure they're faulted in */
> +       for (size_t i = 0; i < size; i += 4096)
> +               mem[i] = mem[i];
> +
> +       /* Keep memory alive for parent to reclaim and check stats */
> +       pause();
> +       free(mem);
> +       return 0;
> +}
> +
> +static long get_zswap_incomp(const char *cgroup)
> +{
> +       return cg_read_key_long(cgroup, "memory.stat", "zswap_incomp ");
> +}
> +
> +/*
> + * Test that incompressible pages (random data) are tracked by zswap_incomp.
> + *
> + * Since incompressible pages stored in zswap are charged at full PAGE_SIZE
> + * (no memory savings), we cannot rely on memory.max pressure to push them
> + * into zswap. Instead, we allocate random data within memory.max, then use
> + * memory.reclaim to proactively push pages into zswap while checking the stat
> + * before the child exits (zswap_incomp is a gauge that decreases on free).

I wonder if we can do MADV_PAGEOUT? Anyway, I'm fine with memory.reclaim too.

> + */
> +static int test_zswap_incompressible(const char *root)
> +{
> +       int ret = KSFT_FAIL;
> +       char *test_group;
> +       long zswap_incomp;
> +       pid_t child_pid;
> +       int child_status;
> +
> +       test_group = cg_name(root, "zswap_incompressible_test");
> +       if (!test_group)
> +               goto out;
> +       if (cg_create(test_group))
> +               goto out;
> +       if (cg_write(test_group, "memory.max", "32M"))
> +               goto out;
> +
> +       child_pid = cg_run_nowait(test_group, allocate_random_and_wait,
> +                                 (void *)MB(4));
> +       if (child_pid < 0)
> +               goto out;
> +
> +       /* Wait for child to finish allocating */
> +       usleep(500000);
> +
> +       /* Proactively reclaim to push random pages into zswap */
> +       cg_write_numeric(test_group, "memory.reclaim", MB(4));
> +
> +       zswap_incomp = get_zswap_incomp(test_group);
> +       if (zswap_incomp <= 0) {
> +               ksft_print_msg("zswap_incomp not increased: %ld\n", zswap_incomp);
> +               goto out_kill;
> +       }
> +
> +       ret = KSFT_PASS;
> +
> +out_kill:
> +       kill(child_pid, SIGTERM);
> +       waitpid(child_pid, &child_status, 0);
> +out:
> +       cg_destroy(test_group);
> +       free(test_group);
> +       return ret;
> +}

LGTM :)

Acked-by: Nhat Pham <nphamcs@gmail.com>