linux-kernel - Re: [PATCH v2 5/5] zsmalloc: Implement writeback mechanism for zsmalloc

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Message-ID: <Y2HuhiyorAHFO+Ss@google.com>
Date:   Wed, 2 Nov 2022 13:13:58 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Nhat Pham <nphamcs@...il.com>
Cc:     akpm@...ux-foundation.org, hannes@...xchg.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, minchan@...nel.org,
        ngupta@...are.org, senozhatsky@...omium.org, sjenning@...hat.com,
        ddstreet@...e.org, vitaly.wool@...sulko.com
Subject: Re: [PATCH v2 5/5] zsmalloc: Implement writeback mechanism for
 zsmalloc

On (22/10/27 11:27), Nhat Pham wrote:
> +
> +static int zs_zpool_shrink(void *pool, unsigned int pages,
> +			unsigned int *reclaimed)
> +{
> +	unsigned int total = 0;
> +	int ret = -EINVAL;
> +
> +	while (total < pages) {
> +		ret = zs_reclaim_page(pool, 8);
> +		if (ret < 0)
> +			break;
> +		total++;
> +	}
> +
> +	if (reclaimed)
> +		*reclaimed = total;
> +
> +	return ret;
> +}

The name collides with shrinker callbacks (compaction). That's a bit
confusing, took me some time.

> @@ -482,6 +504,7 @@ static struct zpool_driver zs_zpool_driver = {
>  	.malloc_support_movable = true,
>  	.malloc =		  zs_zpool_malloc,
>  	.free =			  zs_zpool_free,
> +	.shrink =     zs_zpool_shrink,
>  	.map =			  zs_zpool_map,
>  	.unmap =		  zs_zpool_unmap,
>  	.total_size =		  zs_zpool_total_size,
> @@ -955,6 +978,21 @@ static int trylock_zspage(struct zspage *zspage)
>  	return 0;
>  }

[..]

> +#ifdef CONFIG_ZPOOL
> +static int zs_reclaim_page(struct zs_pool *pool, unsigned int retries)
> +{
> +	int i, obj_idx, ret = 0;
> +	unsigned long handle;
> +	struct zspage *zspage;
> +	struct page *page;
> +	enum fullness_group fullness;
> +
> +	/* Lock LRU and fullness list */
> +	spin_lock(&pool->lock);
> +	if (!pool->ops || !pool->ops->evict || list_empty(&pool->lru) ||

You don't need pool->lock for pool->ops/pool->ops->evict checks.

But, more importantly, I don't understand why is it even checked here?
Why do we use ops->evict? Why cannot we call into zsmalloc evict
directly? All of these are statically defined in zsmalloc, just don't
provide .shrink if !define CONFIG_ZPOOL? Under what circumstances
zsmalloc can provide .shrink but no ->evict?