Re: [PATCH v4 3/5] mm/zswap: Implement proactive writeback

[Yosry Ahmed <yosry@kernel.org>]

On Thu, Jun 18, 2026 at 12:48:55PM +0800, Hao Jia wrote:
> From: Hao Jia <jiahao1@lixiang.com>
> 
> 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 10MB of compressed data from zswap to the backing swap
>   echo "10M zswap_writeback_only" > memory.reclaim
> 
> Note that the actual amount of compressed data written back may be less
> than requested due to the zswap second-chance algorithm: referenced
> entries are rotated on the LRU on the first encounter and only written
> back on a second pass. If fewer bytes are written back than requested,
> -EAGAIN is returned, matching the existing memory.reclaim semantics.
> 
> Internally, extend user_proactive_reclaim() to parse the new
> "zswap_writeback_only" token and invoke the dedicated handler
> zswap_proactive_writeback(). This handler reuses
> zswap_try_to_writeback() to walk the target memcg subtree, draining
> per-node zswap LRUs through list_lru_walk_one() with the
> shrink_memcg_cb() callback.

I won't comment on the memcg interface as this is more-or-less a
placeholder until an interface is finalized.

> 
> Suggested-by: Yosry Ahmed <yosry@kernel.org>
> Suggested-by: Nhat Pham <nphamcs@gmail.com>
> Signed-off-by: Hao Jia <jiahao1@lixiang.com>
[..]
> diff --git a/mm/zswap.c b/mm/zswap.c
> index e29f8a61412d..28200552dde3 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1423,6 +1423,27 @@ static struct mem_cgroup *zswap_iter_global(void)
>  	return memcg;
>  }
>  
> +/*
> + * Local iteration uses a local cursor to select from online memcgs
> + * under @root in a round-robin fashion.
> + *
> + * Pass the previous return value as @prev to advance the round-robin
> + * iteration, or pass NULL to start a new walk. If exiting early before
> + * the iteration completes, the caller must call mem_cgroup_iter_break()
> + * to release the cursor reference.
> + */
> +static struct mem_cgroup *zswap_iter_local(struct mem_cgroup *root,
> +					   struct mem_cgroup *prev)
> +{
> +	struct mem_cgroup *memcg;
> +
> +	do {
> +		memcg = mem_cgroup_iter(root, prev, NULL);
> +		prev = memcg;
> +	} while (memcg && !mem_cgroup_tryget_online(memcg));
> +	return memcg;
> +}
> +
>  /*
>   * Walk the memcg tree and write back zswap pages until the
>   * (lower_pages, upper_pages) window closes, or abort encounter
> @@ -1430,16 +1451,23 @@ static struct mem_cgroup *zswap_iter_global(void)
>   * - No writeback-candidate memcgs found in a memcg tree walk.
>   * - Shrinking a writeback-candidate memcg failed.
>   *
> - * For shrink_worker(), it passes lower=thr and upper=zswap_total_pages().
> - * The @upper limit is refreshed in each iteration by re-evaluating
> - * zswap_total_pages(), and the window closes once the total falls
> - * below the threshold.
> + * For shrink_worker() (proactive=false), it passes lower=thr and
> + * upper=zswap_total_pages(). The @upper limit is refreshed in each
> + * iteration by re-evaluating zswap_total_pages(), and the window
> + * closes once the total falls below the threshold.
> + *
> + * For zswap_proactive_writeback() (proactive=true), it passes lower=0
> + * and upper=nr_to_writeback. The @lower limit is advanced by the
> + * compressed bytes written back via shrink_memcg(). The window closes
> + * once @nr_to_writeback pages of compressed data have been written back.
>   */
> -static void zswap_try_to_writeback(unsigned long lower_pages,
> -				   unsigned long upper_pages)
> +static int zswap_try_to_writeback(struct mem_cgroup *memcg,
> +				  unsigned long lower_pages,
> +				  unsigned long upper_pages, bool proactive)

As I mentiond in the previous patch, this is the wrong abstraction. The
function is extremely tighyl-coupled to the callers, and needing to
pass in things like proactive makes it even worse.

It should be limited to reclaiming one batch of pages from a memcg, and
the retry logic. Everything else (memcg iteration logic, scan goal
checks) should be in the caller.

[..]  
>  static void shrink_worker(struct work_struct *w)
> @@ -1490,7 +1536,7 @@ static void shrink_worker(struct work_struct *w)
>  	/* Reclaim down to the accept threshold */
>  	thr = zswap_accept_thr_pages();
>  
> -	zswap_try_to_writeback(thr, zswap_total_pages());
> +	zswap_try_to_writeback(NULL, thr, zswap_total_pages(), false);
>  }
>  
>  /*********************************
> @@ -1736,6 +1782,19 @@ int zswap_load(struct folio *folio)
>  	return 0;
>  }
>  
> +int zswap_proactive_writeback(struct mem_cgroup *memcg,
> +			      unsigned long nr_to_writeback)
> +{
> +	if (!memcg)
> +		return -EINVAL;
> +	if (!mem_cgroup_zswap_writeback_enabled(memcg))
> +		return -EINVAL;
> +	if (!nr_to_writeback)
> +		return 0;
> +
> +	return zswap_try_to_writeback(memcg, 0, nr_to_writeback, true);

The memcg loop should be here, together with a check on the written
bytes to check if the reclaim goal was achieved. I think nr_to_writeback
is also very confusing, it's really the reclaim target in bytes divided
by PAGE_SIZE. I think you need to pass in the number of bytes to
reclaim/writeback directly.

> +}
> +
>  void zswap_invalidate(swp_entry_t swp)
>  {
>  	pgoff_t offset = swp_offset(swp);
> -- 
> 2.34.1
>