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Message-ID: <324c25c9-fe86-4d82-b4e2-5f3ad76031c7@suse.cz>
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
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