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Message-ID: <r3dzlbqyvhaho5zuac7eba6pxz47zy3cz4lopxza3ls3ibadlh@6evm5aryyuxp>
Date: Fri, 5 Sep 2025 18:53:15 +0000
From: Yosry Ahmed <yosry.ahmed@...ux.dev>
To: Johannes Weiner <hannes@...xchg.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Nhat Pham <nphamcs@...il.com>, Chengming Zhou <zhouchengming@...edance.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] mm: zswap: interact directly with zsmalloc
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@...xchg.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?
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