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

[SeongJae Park <sj@kernel.org>]

On Fri,  6 Feb 2026 15:22:16 +0800 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

Nice test.  This is also testing the functionality of zswap's incompressible
page handling.

> 
> Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>

Reviewed-by: SeongJae Park <sj@kernel.org>

> ---
>  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)

Nit.  I show test_zswapin() is using PAGE_SIZE.  Maybe the above code can also
use it?

> +		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).
> + */
> +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);

We might be better to revisit here in future to avoid racy test results.  But
this seems good enough for now.

> +
> +	/* 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;
> +}
> +
>  #define T(x) { x, #x }
>  struct zswap_test {
>  	int (*fn)(const char *root);
> @@ -586,6 +681,7 @@ struct zswap_test {
>  	T(test_zswap_writeback_disabled),
>  	T(test_no_kmem_bypass),
>  	T(test_no_invasive_cgroup_shrink),
> +	T(test_zswap_incompressible),
>  };
>  #undef T
>  
> -- 
> 2.43.0


Thanks,
SJ

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