Hey Nhat,
I have a few more comments, sorry for not catching everything the first
time around.
Adding Roman to CC.
On Wed, Jan 31, 2024 at 07:27:18PM -0800, Nhat Pham wrote:
> Add a selftest to cover the zswapin code path, allocating more memory
> than the cgroup limit to trigger swapout/zswapout, then reading the
> pages back in memory several times. This is inspired by a recently
> encountered kernel crash on the zswapin path in our internal kernel,
> which went undetected because of a lack of test coverage for this path.
>
> Add a selftest to verify that when memory.zswap.max = 0, no pages can go
> to the zswap pool for the cgroup.
>
> Suggested-by: Rik van Riel <riel@xxxxxxxxxxx>
> Suggested-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> Signed-off-by: Nhat Pham <nphamcs@xxxxxxxxx>
> ---
> tools/testing/selftests/cgroup/test_zswap.c | 97 +++++++++++++++++++++
> 1 file changed, 97 insertions(+)
>
> diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c
> index 32ce975b21d1..14d1f18f1098 100644
> --- a/tools/testing/selftests/cgroup/test_zswap.c
> +++ b/tools/testing/selftests/cgroup/test_zswap.c
> @@ -60,6 +60,27 @@ static long get_zswpout(const char *cgroup)
> return cg_read_key_long(cgroup, "memory.stat", "zswpout ");
> }
>
> +static int allocate_bytes_and_read(const char *cgroup, void *arg)
I think allocate_and_read_bytes() is easier to read, but I don't feel
strongly about it.
> +{
> + size_t size = (size_t)arg;
> + char *mem = (char *)malloc(size);
> + int ret = 0;
> +
> + if (!mem)
> + return -1;
> + for (int i = 0; i < size; i += 4095)
> + mem[i] = 'a';
cgroup_util.h defines PAGE_SIZE, see alloc_anon() for example.
On that note, alloc_anon() is awfully close to allocate_bytes() below,
perhaps we should consolidate them. The only difference I see is that
alloc_anon() does not check for the allocation failure, but a lot of
functions in cgroup_helpers.c don't, so it seems intentional for
simplification.
> +
> + /* go through the allocated memory to (z)swap in and out pages */
> + for (int i = 0; i < size; i += 4095) {
> + if (mem[i] != 'a')
> + ret = -1;
> + }
> +
> + free(mem);
> + return ret;
> +}
> +
> static int allocate_bytes(const char *cgroup, void *arg)
> {
> size_t size = (size_t)arg;
> @@ -133,6 +154,80 @@ static int test_zswap_usage(const char *root)
> return ret;
> }
>
> +/*
> + * Check that when memory.zswap.max = 0, no pages can go to the zswap pool for
> + * the cgroup.
> + */
> +static int test_swapin_nozswap(const char *root)
> +{
> + int ret = KSFT_FAIL;
> + char *test_group;
> + long zswpout;
> +
> + /* Set up */
I think this comment is unnecessary.
> + test_group = cg_name(root, "no_zswap_test");
> +
> + if (!test_group)
> + goto out;
> + if (cg_create(test_group))
> + goto out;
> + if (cg_write(test_group, "memory.max", "8M"))
> + goto out;
> + /* Disable zswap */
I think this comment is unnecessary.
> + if (cg_write(test_group, "memory.zswap.max", "0"))
> + goto out;
> +
> + /* Allocate and read more than memory.max to trigger swapin */
> + if (cg_run(test_group, allocate_bytes_and_read, (void *)MB(32)))
> + goto out;
> +
> + /* Verify that no zswap happened */
If we want to be really meticulous, we can verify that we did swap out,
but not to zswap. IOW, we can check memory.swap.current or something.
> + zswpout = get_zswpout(test_group);
> + if (zswpout < 0) {
> + ksft_print_msg("Failed to get zswpout\n");
> + goto out;
> + } else if (zswpout > 0) {
nit: This can be a separate if condition, I think it would be more
inline with the style of separate consecutive if blocks we are
following.
> + ksft_print_msg(
> + "Pages should not go to zswap when memory.zswap.max = 0\n");
We can probably avoid the line break with something more concise, for
example:
"zswapout > 0 when zswap is disabled"
or "zswapout > 0 when memory.zswap.max = 0"
> + goto out;
> + }
> + ret = KSFT_PASS;
> +
> +out:
> + cg_destroy(test_group);
> + free(test_group);
> + return ret;
> +}
> +
> +/* Simple test to verify the (z)swapin code paths */
> +static int test_zswapin_no_limit(const char *root)
I think test_zswapin() is enough to be distinct from
test_swapin_nozswap(). The limit is not a factor here AFAICT.
> +{
> + int ret = KSFT_FAIL;
> + char *test_group;
> +
> + /* Set up */
I think this comment is unnecessary.
> + test_group = cg_name(root, "zswapin_test");
> + if (!test_group)
> + goto out;
> + if (cg_create(test_group))
> + goto out;
> + if (cg_write(test_group, "memory.max", "8M"))
> + goto out;
> + if (cg_write(test_group, "memory.zswap.max", "max"))
> + goto out;
> +
> + /* Allocate and read more than memory.max to trigger (z)swap in */
> + if (cg_run(test_group, allocate_bytes_and_read, (void *)MB(32)))
> + goto out;
We should probably check for a positive zswapin here, no?
> +
> + ret = KSFT_PASS;
> +
> +out:
> + cg_destroy(test_group);
> + free(test_group);
> + return ret;
> +}
> +
> /*
> * When trying to store a memcg page in zswap, if the memcg hits its memory
> * limit in zswap, writeback should affect only the zswapped pages of that
> @@ -309,6 +404,8 @@ struct zswap_test {
> const char *name;
> } tests[] = {
> T(test_zswap_usage),
> + T(test_swapin_nozswap),
> + T(test_zswapin_no_limit),
> T(test_no_kmem_bypass),
> T(test_no_invasive_cgroup_shrink),
> };
> --
> 2.39.3
