netdev - Re: [PATCH 00/14] replace call_rcu by kfree_rcu for simple kmem_cache

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Date: Mon, 17 Jun 2024 16:37:20 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: "Jason A. Donenfeld" <Jason@...c4.com>,
 Uladzislau Rezki <urezki@...il.com>
Cc: "Paul E. McKenney" <paulmck@...nel.org>, Jakub Kicinski
 <kuba@...nel.org>, Julia Lawall <Julia.Lawall@...ia.fr>,
 linux-block@...r.kernel.org, kernel-janitors@...r.kernel.org,
 bridge@...ts.linux.dev, linux-trace-kernel@...r.kernel.org,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, kvm@...r.kernel.org,
 linuxppc-dev@...ts.ozlabs.org, "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 Nicholas Piggin <npiggin@...il.com>, netdev@...r.kernel.org,
 wireguard@...ts.zx2c4.com, linux-kernel@...r.kernel.org,
 ecryptfs@...r.kernel.org, Neil Brown <neilb@...e.de>,
 Olga Kornievskaia <kolga@...app.com>, Dai Ngo <Dai.Ngo@...cle.com>,
 Tom Talpey <tom@...pey.com>, linux-nfs@...r.kernel.org,
 linux-can@...r.kernel.org, Lai Jiangshan <jiangshanlai@...il.com>,
 netfilter-devel@...r.kernel.org, coreteam@...filter.org
Subject: Re: [PATCH 00/14] replace call_rcu by kfree_rcu for simple
 kmem_cache_free callback

On 6/14/24 9:33 PM, Jason A. Donenfeld wrote:
> On Fri, Jun 14, 2024 at 02:35:33PM +0200, Uladzislau Rezki wrote:
>> +	/* Should a destroy process be deferred? */
>> +	if (s->flags & SLAB_DEFER_DESTROY) {
>> +		list_move_tail(&s->list, &slab_caches_defer_destroy);
>> +		schedule_delayed_work(&slab_caches_defer_destroy_work, HZ);
>> +		goto out_unlock;
>> +	}
> 
> Wouldn't it be smoother to have the actual kmem_cache_free() function
> check to see if it's been marked for destruction and the refcount is
> zero, rather than polling every one second? I mentioned this approach
> in: https://lore.kernel.org/all/Zmo9-YGraiCj5-MI@zx2c4.com/ -
> 
>     I wonder if the right fix to this would be adding a `should_destroy`
>     boolean to kmem_cache, which kmem_cache_destroy() sets to true. And
>     then right after it checks `if (number_of_allocations == 0)
>     actually_destroy()`, and likewise on each kmem_cache_free(), it
>     could check `if (should_destroy && number_of_allocations == 0)
>     actually_destroy()`. 

I would prefer not to affect the performance of kmem_cache_free() by doing
such checks, if possible. Ideally we'd have a way to wait/poll for the
kfree_rcu() "grace period" expiring even with the batching that's
implemented there. Even if it's pesimistically long to avoid affecting
kfree_rcu() performance. The goal here is just to print the warnings if
there was a leak and the precise timing of them shouldn't matter. The owning
module could be already unloaded at that point? I guess only a kunit test
could want to be synchronous and then it could just ask for
kmem_cache_free() to wait synchronously.

> Jason