Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:18 as permitted sender) client-ip=2620:137:e000::1:18;
Date:   Wed, 8 Jun 2022 14:23:45 +0200 (CEST)
From:   Christoph Lameter <cl@gentwo.de>
To:     Rongwei Wang <rongwei.wang@linux.alibaba.com>
cc:     David Rientjes <rientjes@google.com>, songmuchun@bytedance.com,
        Hyeonggon Yoo <42.hyeyoo@gmail.com>, akpm@linux-foundation.org,
        vbabka@suse.cz, roman.gushchin@linux.dev, iamjoonsoo.kim@lge.com,
        penberg@kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/3] mm/slub: fix the race between validate_slab and
 slab_free
In-Reply-To: <29723aaa-5e28-51d3-7f87-9edf0f7b9c33@linux.alibaba.com>
Message-ID: <alpine.DEB.2.22.394.2206081417370.465021@gentwo.de>
References: <20220529081535.69275-1-rongwei.wang@linux.alibaba.com> <YpNa4tB/jfW3MDyi@n2.us-central1-a.c.spheric-algebra-350919.internal> <ac9ba68f-9ee2-1611-9ff8-b486ed9c4df0@google.com> <alpine.DEB.2.22.394.2206021712530.2924@gentwo.de>
 <9794df4f-3ffe-4e99-0810-a1346b139ce8@linux.alibaba.com> <alpine.DEB.2.22.394.2206071411460.375438@gentwo.de> <29723aaa-5e28-51d3-7f87-9edf0f7b9c33@linux.alibaba.com>
User-Agent: Alpine 2.22 (DEB 394 2020-01-19)
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Precedence: bulk

On Wed, 8 Jun 2022, Rongwei Wang wrote:

> If available, I think document the issue and warn this incorrect behavior is
> OK. But it still prints a large amount of confusing messages, and disturbs us?

Correct it would be great if you could fix this in a way that does not
impact performance.

> > are current operations on the slab being validated.
> And I am trying to fix it in following way. In a short, these changes only
> works under the slub debug mode, and not affects the normal mode (I'm not
> sure). It looks not elegant enough. And if all approve of this way, I can
> submit the next version.


>
> Anyway, thanks for your time:).
> -wrw
>
> @@ -3304,7 +3300,7 @@ static void __slab_free(struct kmem_cache *s,
struct
> slab *slab,
>
>  {
>         void *prior;
> -       int was_frozen;
> +       int was_frozen, to_take_off = 0;
>         struct slab new;

to_take_off has the role of !n ? Why is that needed?

> -       do {
> -               if (unlikely(n)) {
> +               spin_lock_irqsave(&n->list_lock, flags);
> +               ret = free_debug_processing(s, slab, head, tail, cnt, addr);

Ok so the idea is to take the lock only if kmem_cache_debug. That looks
ok. But it still adds a number of new branches etc to the free loop.

Some performance tests would be useful.