Re: [PATCH v6 7/8] selftest/cgroup: fix zswap attempt_writeback() on 64K pagesize system

[Nhat Pham]

On Wed, Apr 1, 2026 at 11:38 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/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.

Makes sense to me. We should scale the test by PAGE_SIZE.

Acked-by: Nhat Pham <nphamcs@xxxxxxxxx>