Re: [PATCH v2 7/7] selftest/cgroup: fix zswap attempt_writeback() on 64K pagesize system

[Yosry Ahmed]

On Wed, Mar 11, 2026 at 9:07 PM 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.
>
> 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
> 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 <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>

Acked-by: Yosry Ahmed <yosry@xxxxxxxxxx>