RE: [PATCH v13 19/22] mm: zswap: Per-CPU acomp_ctx resources exist from pool creation to deletion.

["Sridhar, Kanchana P" <kanchana.p.sridhar@intel.com>]

> -----Original Message-----
> From: Yosry Ahmed <yosry.ahmed@linux.dev>
> Sent: Friday, December 12, 2025 10:44 AM
> To: Sridhar, Kanchana P <kanchana.p.sridhar@intel.com>
> Cc: linux-kernel@vger.kernel.org; linux-mm@kvack.org;
> hannes@cmpxchg.org; nphamcs@gmail.com; chengming.zhou@linux.dev;
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> ying.huang@linux.alibaba.com; akpm@linux-foundation.org;
> senozhatsky@chromium.org; sj@kernel.org; kasong@tencent.com; linux-
> crypto@vger.kernel.org; herbert@gondor.apana.org.au;
> davem@davemloft.net; clabbe@baylibre.com; ardb@kernel.org;
> ebiggers@google.com; surenb@google.com; Accardi, Kristen C
> <kristen.c.accardi@intel.com>; Gomes, Vinicius <vinicius.gomes@intel.com>;
> Feghali, Wajdi K <wajdi.k.feghali@intel.com>; Gopal, Vinodh
> <vinodh.gopal@intel.com>
> Subject: Re: [PATCH v13 19/22] mm: zswap: Per-CPU acomp_ctx resources
> exist from pool creation to deletion.
> 
> On Fri, Dec 12, 2025 at 06:17:07PM +0000, Sridhar, Kanchana P wrote:
> > >
> > > >  	ret =
> > > cpuhp_state_add_instance(CPUHP_MM_ZSWP_POOL_PREPARE,
> > > >  				       &pool->node);
> > > >  	if (ret)
> > > > -		goto error;
> > > > +		goto ref_fail;
> > >
> > > IIUC we shouldn't call cpuhp_state_remove_instance() on failure, we
> > > probably should add a new label.
> >
> > In this case we should because it is part of the pool creation failure
> > handling flow, at the end of which, the pool will be deleted.
> 
> What I mean is, when cpuhp_state_add_instance() fails we goto ref_fail
> which will call cpuhp_state_remove_instance(). But the current code does
> not call cpuhp_state_remove_instance() if cpuhp_state_add_instance()
> fails.

I see what you mean. The current mainline code does not call
cpuhp_state_remove_instance() if cpuhp_state_add_instance() fails, because
the cpuhotplug code will call the teardown callback in this case.

In this patch, I do need to call cpuhp_state_remove_instance() and
acomp_ctx_dealloc() in this case because there is no teardown callback
being registered.

> 
> >
> > >
> > > >
> > > >  	/* being the current pool takes 1 ref; this func expects the
> > > >  	 * caller to always add the new pool as the current pool
> > > > @@ -292,6 +313,9 @@ static struct zswap_pool
> *zswap_pool_create(char
> > > *compressor)
> > > >
> > > >  ref_fail:
> > > >  	cpuhp_state_remove_instance(CPUHP_MM_ZSWP_POOL_PREPARE,
> > > &pool->node);
> > > > +
> > > > +	for_each_possible_cpu(cpu)
> > > > +		acomp_ctx_dealloc(per_cpu_ptr(pool->acomp_ctx, cpu));
> > > >  error:
> > > >  	if (pool->acomp_ctx)
> > > >  		free_percpu(pool->acomp_ctx);
> > > > @@ -322,9 +346,15 @@ static struct zswap_pool
> > > *__zswap_pool_create_fallback(void)
> > > >
> > > >  static void zswap_pool_destroy(struct zswap_pool *pool)
> > > >  {
> > > > +	int cpu;
> > > > +
> > > >  	zswap_pool_debug("destroying", pool);
> > > >
> > > >  	cpuhp_state_remove_instance(CPUHP_MM_ZSWP_POOL_PREPARE,
> > > &pool->node);
> > > > +
> > > > +	for_each_possible_cpu(cpu)
> > > > +		acomp_ctx_dealloc(per_cpu_ptr(pool->acomp_ctx, cpu));
> > > > +
> > > >  	free_percpu(pool->acomp_ctx);
> > > >
> > > >  	zs_destroy_pool(pool->zs_pool);
> > > > @@ -736,39 +766,35 @@ static int
> zswap_cpu_comp_prepare(unsigned int
> > > cpu, struct hlist_node *node)
> > > >  {
> > > >  	struct zswap_pool *pool = hlist_entry(node, struct zswap_pool,
> > > node);
> > > >  	struct crypto_acomp_ctx *acomp_ctx = per_cpu_ptr(pool-
> > > >acomp_ctx, cpu);
> > > > -	struct crypto_acomp *acomp = NULL;
> > > > -	struct acomp_req *req = NULL;
> > > > -	u8 *buffer = NULL;
> > > > -	int ret;
> > > > +	int ret = -ENOMEM;
> > > >
> > > > -	buffer = kmalloc_node(PAGE_SIZE, GFP_KERNEL, cpu_to_node(cpu));
> > > > -	if (!buffer) {
> > > > -		ret = -ENOMEM;
> > > > -		goto fail;
> > > > -	}
> > > > +	/*
> > > > +	 * To handle cases where the CPU goes through online-offline-online
> > > > +	 * transitions, we return if the acomp_ctx has already been initialized.
> > > > +	 */
> > > > +	if (!IS_ERR_OR_NULL(acomp_ctx->acomp))
> > > > +		return 0;
> > >
> > > Is it possible for acomp_ctx->acomp to be an ERR value here? If it is,
> > > then zswap initialization should have failed. Maybe WARN_ON_ONCE() for
> > > that case?
> >
> > This is checking for a valid acomp_ctx->acomp using the same criteria
> > being uniformly used in acomp_ctx_dealloc(). This check is necessary to
> > handle the case where the CPU goes through online-offline-online state
> > transitions.
> 
> I think I am confused. I thought now we don't free this on CPU offline,
> so either it's NULL because this is the first time we initialize it on
> this CPU, or it is allocated.

Yes, this is correct.

> If it is an ERR value, then the pool
> creation should have failed and we wouldn't be calling this again on CPU
> online.
> 
> In other words, what scenario do we expect to legitimately see an ERR
> value here?

I am using "(!IS_ERR_OR_NULL(acomp_ctx->acomp)" as a check for the
acomp being allocated already. I could instead have used "if (acomp_ctx->acomp)",
but use the former to be consistent with patch 20/22.

I cannot think of a scenario where we can expect an ERR value here.

Thanks,
Kanchana