Re: [PATCH v7 7/8] selftest/cgroup: fix zswap attempt_writeback() on 64K pagesize system

[Michal Koutný]

On Fri, Apr 24, 2026 at 12:00:58PM +0800, Li Wang <li.wang@xxxxxxxxx> 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/4 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.
> 
> Fix this by:
> - Always allocating 1024 pages regardless of page size. This ensures
>   enough anonymous pages to reliably populate zswap and trigger
>   writeback, while keeping the original 4M allocation on 4K systems.
> - Setting zswap.max to zswap_usage/4 instead of zswap_usage/2 to
>   create stronger writeback pressure, ensuring reclaim reliably
>   triggers writeback even on large page size systems.
> 
> === 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 <li.wang@xxxxxxxxx>
> 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>
> Acked-by: Yosry Ahmed <yosry@xxxxxxxxxx>
> Acked-by: Nhat Pham <nphamcs@xxxxxxxxx>
> ---
>  tools/testing/selftests/cgroup/test_zswap.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)

Reviewed-by: Michal Koutný <mkoutny@xxxxxxxx>

Attachment: signature.asc
Description: PGP signature