selftests/cgroup: add test for zswap incompressible pages

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

Link: https://lkml.kernel.org/r/20260213071827.5688-3-jiayuan.chen@linux.dev
Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Yosry Ahmed <yosry.ahmed@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/cgroup/test_zswap.c

@@ -5,6 +5,8 @@
 #include <unistd.h>
 #include <stdio.h>
 #include <signal.h>
+#include <errno.h>
+#include <fcntl.h>
 #include <sys/sysinfo.h>
 #include <string.h>
 #include <sys/wait.h>
@@ -574,6 +576,139 @@ out:
 	return ret;
 }
 
+struct incomp_child_args {
+	size_t size;
+	int pipefd[2];
+	int madvise_ret;
+	int madvise_errno;
+};
+
+static int allocate_random_and_wait(const char *cgroup, void *arg)
+{
+	struct incomp_child_args *values = arg;
+	size_t size = values->size;
+	char *mem;
+	int fd;
+	ssize_t n;
+
+	close(values->pipefd[0]);
+
+	mem = mmap(NULL, size, PROT_READ | PROT_WRITE,
+		   MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+	if (mem == MAP_FAILED)
+		return -1;
+
+	/* Fill with random data from /dev/urandom - incompressible */
+	fd = open("/dev/urandom", O_RDONLY);
+	if (fd < 0) {
+		munmap(mem, size);
+		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 += PAGE_SIZE)
+		mem[i] = mem[i];
+
+	/* Use MADV_PAGEOUT to push pages into zswap */
+	values->madvise_ret = madvise(mem, size, MADV_PAGEOUT);
+	values->madvise_errno = errno;
+
+	/* Notify parent that allocation and pageout are done */
+	write(values->pipefd[1], "x", 1);
+	close(values->pipefd[1]);
+
+	/* Keep memory alive for parent to check stats */
+	pause();
+	munmap(mem, size);
+	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.
+ *
+ * The child process allocates random data within memory.max, then uses
+ * MADV_PAGEOUT to push pages into zswap. The parent waits on a pipe for
+ * the child to finish, then checks the zswap_incomp 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;
+	struct incomp_child_args *values;
+	char *test_group;
+	long zswap_incomp;
+	pid_t child_pid;
+	int child_status;
+	char buf;
+
+	values = mmap(0, sizeof(struct incomp_child_args), PROT_READ |
+			PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
+	if (values == MAP_FAILED)
+		return KSFT_FAIL;
+
+	if (pipe(values->pipefd)) {
+		munmap(values, sizeof(struct incomp_child_args));
+		return KSFT_FAIL;
+	}
+
+	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;
+
+	values->size = MB(4);
+	child_pid = cg_run_nowait(test_group, allocate_random_and_wait, values);
+	if (child_pid < 0)
+		goto out;
+
+	close(values->pipefd[1]);
+
+	/* Wait for child to finish allocating and pageout */
+	read(values->pipefd[0], &buf, 1);
+	close(values->pipefd[0]);
+
+	zswap_incomp = get_zswap_incomp(test_group);
+	if (zswap_incomp <= 0) {
+		long zswpout = get_zswpout(test_group);
+		long zswapped = cg_read_key_long(test_group, "memory.stat", "zswapped ");
+		long zswap_b = cg_read_key_long(test_group, "memory.stat", "zswap ");
+
+		ksft_print_msg("zswap_incomp not increased: %ld\n", zswap_incomp);
+		ksft_print_msg("debug: zswpout=%ld zswapped=%ld zswap_b=%ld\n",
+			       zswpout, zswapped, zswap_b);
+		ksft_print_msg("debug: madvise ret=%d errno=%d\n",
+			       values->madvise_ret, values->madvise_errno);
+		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);
+	munmap(values, sizeof(struct incomp_child_args));
+	return ret;
+}
+
 #define T(x) { x, #x }
 struct zswap_test {
 	int (*fn)(const char *root);
@@ -586,6 +721,7 @@ 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