Re: [PATCH 1/3] mm/slub: fix the race between validate_slab and slab_free

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
Date: Wed, 8 Jun 2022 14:23:45 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.22.394.2206081417370.465021@gentwo.de> (raw)
In-Reply-To: <29723aaa-5e28-51d3-7f87-9edf0f7b9c33@linux.alibaba.com>

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.