Re: [PATCH 1/3] mm: zswap: interact directly with zsmalloc

[Yosry Ahmed <yosry.ahmed@linux.dev>]

On Fri, Aug 29, 2025 at 05:15:26PM +0100, Johannes Weiner wrote:
> zswap goes through the zpool layer to enable runtime-switching of
> allocator backends for compressed data. However, since zbud and z3fold
> were removed in 6.15, zsmalloc has been the only option available.
> 
> As such, the zpool indirection is unnecessary. Make zswap deal with
> zsmalloc directly. This is comparable to zram, which also directly
> interacts with zsmalloc and has never supported a different backend.
> 
> Note that this does not preclude future improvements and experiments
> with different allocation strategies. Should it become necessary, it's
> possible to provide an alternate implementation for the zsmalloc API,
> selectable at compile time. However, zsmalloc is also rather mature
> and feature rich, with years of widespread production exposure; it's
> encouraged to make incremental improvements rather than fork it.
> 
> In any case, the complexity of runtime pluggability seems excessive
> and unjustified at this time. Switch zswap to zsmalloc to remove the
> last user of the zpool API.
> 
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
[..]
> @@ -315,52 +292,29 @@ static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
>  error:
>  	if (pool->acomp_ctx)
>  		free_percpu(pool->acomp_ctx);
> -	if (pool->zpool)
> -		zpool_destroy_pool(pool->zpool);
> +	if (pool->zs_pool)
> +		zs_destroy_pool(pool->zs_pool);
>  	kfree(pool);
>  	return NULL;
>  }
>  
>  static struct zswap_pool *__zswap_pool_create_fallback(void)
>  {
> -	bool has_comp, has_zpool;
> -
> -	has_comp = crypto_has_acomp(zswap_compressor, 0, 0);
> -	if (!has_comp && strcmp(zswap_compressor,
> -				CONFIG_ZSWAP_COMPRESSOR_DEFAULT)) {
> +	if (!crypto_has_acomp(zswap_compressor, 0, 0) &&
> +	    strcmp(zswap_compressor, CONFIG_ZSWAP_COMPRESSOR_DEFAULT)) {
>  		pr_err("compressor %s not available, using default %s\n",
>  		       zswap_compressor, CONFIG_ZSWAP_COMPRESSOR_DEFAULT);
>  		param_free_charp(&zswap_compressor);
>  		zswap_compressor = CONFIG_ZSWAP_COMPRESSOR_DEFAULT;
> -		has_comp = crypto_has_acomp(zswap_compressor, 0, 0);
> -	}
> -	if (!has_comp) {
> -		pr_err("default compressor %s not available\n",
> -		       zswap_compressor);
> -		param_free_charp(&zswap_compressor);
> -		zswap_compressor = ZSWAP_PARAM_UNSET;
> -	}
> -
> -	has_zpool = zpool_has_pool(zswap_zpool_type);
> -	if (!has_zpool && strcmp(zswap_zpool_type,
> -				 CONFIG_ZSWAP_ZPOOL_DEFAULT)) {
> -		pr_err("zpool %s not available, using default %s\n",
> -		       zswap_zpool_type, CONFIG_ZSWAP_ZPOOL_DEFAULT);
> -		param_free_charp(&zswap_zpool_type);
> -		zswap_zpool_type = CONFIG_ZSWAP_ZPOOL_DEFAULT;
> -		has_zpool = zpool_has_pool(zswap_zpool_type);
> -	}
> -	if (!has_zpool) {
> -		pr_err("default zpool %s not available\n",
> -		       zswap_zpool_type);
> -		param_free_charp(&zswap_zpool_type);
> -		zswap_zpool_type = ZSWAP_PARAM_UNSET;
> +		if (!crypto_has_acomp(zswap_compressor, 0, 0)) {
> +			pr_err("default compressor %s not available\n",
> +			       zswap_compressor);
> +			zswap_compressor = ZSWAP_PARAM_UNSET;
> +			return NULL;
> +		}

Hmm it seems like there may be a change of behavior here. If
zswap_compressor == CONFIG_ZSWAP_COMPRESSOR_DEFAULT at the beginning and
crypto_has_acomp() returns false, the old code will go into the second
if (!has_comp) block, printing an error, freeing the string, and setting
zswap_compressor to ZSWAP_PARAM_UNSET, then we eventually return NULL.

It seems like the new code will just call zswap_pool_create() anyway.

Am I missing something here?