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

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Date: Mon, 17 Jun 2024 18:33:23 +0200
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: Uladzislau Rezki <urezki@...il.com>
Cc: "Paul E. McKenney" <paulmck@...nel.org>, Vlastimil Babka <vbabka@...e.cz>, 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 Mon, Jun 17, 2024 at 6:30 PM Uladzislau Rezki <urezki@...il.com> wrote:
> Here if an "err" is less then "0" means there are still objects
> whereas "is_destroyed" is set to "true" which is not correlated
> with a comment:
>
> "Destruction happens when no objects"

The comment is just poorly written. But the logic of the code is right.

>
> >  out_unlock:
> >       mutex_unlock(&slab_mutex);
> >       cpus_read_unlock();
> > diff --git a/mm/slub.c b/mm/slub.c
> > index 1373ac365a46..7db8fe90a323 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -4510,6 +4510,8 @@ void kmem_cache_free(struct kmem_cache *s, void *x)
> >               return;
> >       trace_kmem_cache_free(_RET_IP_, x, s);
> >       slab_free(s, virt_to_slab(x), x, _RET_IP_);
> > +     if (s->is_destroyed)
> > +             kmem_cache_destroy(s);
> >  }
> >  EXPORT_SYMBOL(kmem_cache_free);
> >
> > @@ -5342,9 +5344,6 @@ static void free_partial(struct kmem_cache *s, struct kmem_cache_node *n)
> >               if (!slab->inuse) {
> >                       remove_partial(n, slab);
> >                       list_add(&slab->slab_list, &discard);
> > -             } else {
> > -                     list_slab_objects(s, slab,
> > -                       "Objects remaining in %s on __kmem_cache_shutdown()");
> >               }
> >       }
> >       spin_unlock_irq(&n->list_lock);
> >
> Anyway it looks like it was not welcome to do it in the kmem_cache_free()
> function due to performance reason.

"was not welcome" - Vlastimil mentioned *potential* performance
concerns before I posted this. I suspect he might have a different
view now, maybe?

Vlastimil, this is just checking a boolean (which could be
unlikely()'d), which should have pretty minimal overhead. Is that
alright with you?

Jason