Re: [PATCH 5/5] selftest/cgroup: fix zswap attempt_writeback() on 64K pagesize system

[Yosry Ahmed]

On Wed, Mar 11, 2026 at 4:06 AM Li Wang <liwang@xxxxxxxxxx> wrote:
>
> In attempt_writeback(), a memsize of 4M only covers 64 pages on 64K
> page size systems. When memory.reclaim is called, the kernel prefers
> reclaiming clean file pages (binary, libc, linker, etc.) over swapping
> anonymous pages. With only 64 pages of anonymous memory, the reclaim
> target can be largely or entirely satisfied by dropping file pages,
> resulting in very few or zero anonymous pages being pushed into zswap.
>
> This causes zswap_usage to be extremely small or zero, making
> zswap_usage/2 insufficient to create meaningful writeback pressure.
> The test then fails because no writeback is triggered.
>
> On 4K page size systems this is not an issue because 4M covers 1024
> pages, and file pages are a small fraction of the reclaim target.
>
> Scale memsize up to 64M on systems with page size larger than 4K, so
> that enough anonymous pages are allocated to reliably populate zswap
> and trigger writeback. The original 4M is kept for 4K page size systems
> to avoid unnecessary memory usage and test runtime.
>
> === Error Log ===
>   # uname -rm
>   6.12.0-211.el10.ppc64le ppc64le
>
>   # getconf PAGESIZE
>   65536
>
>   # ./test_zswap
>   TAP version 13
>   1..7
>   ok 1 test_zswap_usage
>   ok 2 test_swapin_nozswap
>   ok 3 test_zswapin
>   not ok 4 test_zswap_writeback_enabled
>   ...
>
> Signed-off-by: Li Wang <liwang@xxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: Michal Koutný <mkoutny@xxxxxxxx>
> Cc: Muchun Song <muchun.song@xxxxxxxxx>
> Cc: Nhat Pham <nphamcs@xxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx>
> Cc: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> Cc: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> ---
>  tools/testing/selftests/cgroup/test_zswap.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c
> index 30d3fbf6b4fb..2d065184eea4 100644
> --- a/tools/testing/selftests/cgroup/test_zswap.c
> +++ b/tools/testing/selftests/cgroup/test_zswap.c
> @@ -264,7 +264,7 @@ static int test_zswapin(const char *root)
>  static int attempt_writeback(const char *cgroup, void *arg)
>  {
>         long pagesize = sysconf(_SC_PAGESIZE);
> -       size_t memsize = MB(4);
> +       size_t memsize = pagesize > 4096 ? MB(64): MB(4);

pagesize << 10 or pagesize * 1024?

>         char buf[pagesize];
>         long zswap_usage;
>         bool wb_enabled = *(bool *) arg;
> --
> 2.53.0
>