Re: [PATCH v2 2/2] memcg: selftests for memcg stat kfuncs

[Shakeel Butt <shakeel.butt@linux.dev>]

On Wed, Oct 15, 2025 at 12:08:13PM -0700, JP Kobryn wrote:
> Add test coverage for the kfuncs that fetch memcg stats. Using some common
> stats, test before and after scenarios ensuring that the given stat
> increases by some arbitrary amount. The stats selected cover the three
> categories represented by the enums: node_stat_item, memcg_stat_item,
> vm_event_item.
> 
> Since only a subset of all stats are queried, use a static struct made up
> of fields for each stat. Write to the struct with the fetched values when
> the bpf program is invoked and read the fields in the user mode program for
> verification.
> 
> Signed-off-by: JP Kobryn <inwardvessel@gmail.com>
> ---
>  .../testing/selftests/bpf/cgroup_iter_memcg.h |  18 ++
>  .../bpf/prog_tests/cgroup_iter_memcg.c        | 295 ++++++++++++++++++
>  .../selftests/bpf/progs/cgroup_iter_memcg.c   |  61 ++++
>  3 files changed, 374 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/cgroup_iter_memcg.h
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/cgroup_iter_memcg.c
>  create mode 100644 tools/testing/selftests/bpf/progs/cgroup_iter_memcg.c
> 
> diff --git a/tools/testing/selftests/bpf/cgroup_iter_memcg.h b/tools/testing/selftests/bpf/cgroup_iter_memcg.h
> new file mode 100644
> index 000000000000..5f4c6502d9f1
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/cgroup_iter_memcg.h
> @@ -0,0 +1,18 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/* Copyright (c) 2025 Meta Platforms, Inc. and affiliates. */
> +#ifndef __CGROUP_ITER_MEMCG_H
> +#define __CGROUP_ITER_MEMCG_H
> +
> +struct memcg_query {
> +	/* some node_stat_item's */
> +	long nr_anon_mapped;
> +	long nr_shmem;
> +	long nr_file_pages;
> +	long nr_file_mapped;
> +	/* some memcg_stat_item */
> +	long memcg_kmem;
> +	/* some vm_event_item */
> +	long pgfault;
> +};
> +
> +#endif /* __CGROUP_ITER_MEMCG_H */
> diff --git a/tools/testing/selftests/bpf/prog_tests/cgroup_iter_memcg.c b/tools/testing/selftests/bpf/prog_tests/cgroup_iter_memcg.c
> new file mode 100644
> index 000000000000..264dc3c9ec30
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/cgroup_iter_memcg.c
> @@ -0,0 +1,295 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (c) 2025 Meta Platforms, Inc. and affiliates. */
> +#include <test_progs.h>
> +#include <bpf/libbpf.h>
> +#include <bpf/btf.h>
> +#include <fcntl.h>
> +#include <sys/mman.h>
> +#include <unistd.h>
> +#include "cgroup_helpers.h"
> +#include "cgroup_iter_memcg.h"
> +#include "cgroup_iter_memcg.skel.h"
> +
> +int read_stats(struct bpf_link *link)
> +{
> +	int fd, ret = 0;
> +	ssize_t bytes;
> +
> +	fd = bpf_iter_create(bpf_link__fd(link));

Do we need to create a new fd for every new stat read? Is there lseek()
like approach possible here?

> +	if (!ASSERT_OK_FD(fd, "bpf_iter_create"))
> +		return 1;
> +
> +	/*
> +	 * Invoke iter program by reading from its fd. We're not expecting any
> +	 * data to be written by the bpf program so the result should be zero.
> +	 * Results will be read directly through the custom data section
> +	 * accessible through skel->data_query.memcg_query.
> +	 */
> +	bytes = read(fd, NULL, 0);
> +	if (!ASSERT_EQ(bytes, 0, "read fd"))
> +		ret = 1;
> +
> +	close(fd);
> +	return ret;
> +}
> +

[...]

> +static void test_shmem(struct bpf_link *link,
> +		struct memcg_query *memcg_query)
> +{
> +	size_t len;
> +	int fd;
> +	void *map;
> +	long val;
> +
> +	len = sysconf(_SC_PAGESIZE) * 1024;
> +
> +	if (!ASSERT_OK(read_stats(link), "read stats"))
> +		return;
> +
> +	val = memcg_query->nr_shmem;
> +	if (!ASSERT_GE(val, 0, "init shmem val"))
> +		return;
> +
> +	/*
> +	 * Increase memcg shmem usage by creating and writing
> +	 * to a shmem object.
> +	 */
> +	fd = shm_open("/tmp_shmem", O_CREAT | O_RDWR, 0644);
> +	if (!ASSERT_OK_FD(fd, "shm_open"))
> +		return;
> +
> +	if (!ASSERT_OK(ftruncate(fd, len), "ftruncate"))
> +		goto cleanup_fd;
> +
> +	map = mmap(NULL, len, PROT_READ | PROT_WRITE,
> +			MAP_SHARED, fd, 0);

You don't need to mmap(), you can just fallocate() or simply write to
increase shmem stats.

> +	if (!ASSERT_NEQ(map, MAP_FAILED, "mmap shmem"))
> +		goto cleanup_fd;
> +
> +	memset(map, 1, len);
> +
> +	if (!ASSERT_OK(read_stats(link), "read stats"))
> +		goto cleanup_map;
> +
> +	ASSERT_GT(memcg_query->nr_shmem, val, "final shmem value");
> +
> +cleanup_map:
> +	munmap(map, len);
> +cleanup_fd:
> +	close(fd);
> +	shm_unlink("/tmp_shmem");
> +}
> +
> +static void test_kmem(struct bpf_link *link,
> +		struct memcg_query *memcg_query)
> +{
> +	int fds[2];
> +	int err;
> +	ssize_t bytes;
> +	size_t len;
> +	char *buf;
> +	long val;
> +
> +	len = sysconf(_SC_PAGESIZE) * 1024;
> +
> +	if (!ASSERT_OK(read_stats(link), "read stats"))
> +		return;
> +
> +	val = memcg_query->memcg_kmem;
> +	if (!ASSERT_GE(val, 0, "initial kmem val"))
> +		return;
> +
> +	err = pipe2(fds, O_NONBLOCK);

You will need to create a bit more of these cause the kernel memory
stats to be updated due to per-cpu memcg stock caching kmem stats.

> +	if (!ASSERT_OK(err, "pipe"))
> +		return;
> +
> +	buf = malloc(len);
> +	memset(buf, 1, len);
> +	bytes = write(fds[1], buf, len);
> +	if (!ASSERT_GT(bytes, 0, "write"))
> +		goto cleanup;
> +
> +	if (!ASSERT_OK(read_stats(link), "read stats"))
> +		goto cleanup;
> +
> +	ASSERT_GT(memcg_query->memcg_kmem, val, "kmem value");
> +
> +cleanup:
> +	free(buf);
> +	close(fds[0]);
> +	close(fds[1]);
> +}
> +