Re: [PATCH v3 2/4] mm/zswap: Implement proactive writeback

[Nhat Pham]

On Tue, May 26, 2026 at 4:46 AM Hao Jia <jiahao.kernel@xxxxxxxxx> wrote:
>
> From: Hao Jia <jiahao1@xxxxxxxxxxx>
>
> Zswap currently writes back pages to backing swap reactively, triggered
> either by the shrinker or when the pool reaches its size limit. There is
> no mechanism to control the amount of writeback for a specific memory
> cgroup. However, users may want to proactively write back zswap pages,
> e.g., to free up memory for other applications or to prepare for
> memory-intensive workloads.
>
> Introduce a "zswap_writeback_only" key to the memory.reclaim cgroup
> interface. When specified, this key bypasses standard memory reclaim
> and exclusively performs proactive zswap writeback up to the requested
> budget. If omitted, the default reclaim behavior remains unchanged.
>
> Example usage:
>   # Write back 100MB of pages from zswap to the backing swap
>   echo "100M zswap_writeback_only" > memory.reclaim

Hmmm, so this 100MB is the pre-compression size? i.e if this 100 MB
compresses to 25 MB, then you're only freeing 25 MB?

I'm ok-ish with this, but can you document it?

The rest seems solid to me, FWIW. I'll defer to Johannes and Yosry for
opinions on zswap-only proactive reclaim.