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

[Johannes Weiner <hannes@cmpxchg.org>]

On Fri, Sep 05, 2025 at 06:53:15PM +0000, Yosry Ahmed wrote:
> 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?

I don't think that scenario is possible, due to the way the Kconfig
works. Whatever backend I select for CONFIG_ZSWAP_COMPRESSOR_DEFAULT
pulls in the crypto module as built-in/=y. It should always be there.