linux-kernel - Re: [PATCH] slub: fix __kmem_cache_empty for !CONFIG_SLUB

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Date:   Tue, 19 Jun 2018 23:53:57 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, Andrew Morton <akpm@...ux-foundation.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH] slub: fix __kmem_cache_empty for !CONFIG_SLUB_DEBUG

On Tue, Jun 19, 2018 at 11:34 PM Shakeel Butt <shakeelb@...gle.com> wrote:
>
> For !CONFIG_SLUB_DEBUG, SLUB does not maintain the number of slabs
> allocated per node for a kmem_cache. Thus, slabs_node() in
> __kmem_cache_empty() will always return 0. So, in such situation, it is
> required to check per-cpu slabs to make sure if a kmem_cache is empty or
> not.
>
> Please note that __kmem_cache_shutdown() and __kmem_cache_shrink() are
> not affected by !CONFIG_SLUB_DEBUG as they call flush_all() to clear
> per-cpu slabs.
>
> Fixes: f9e13c0a5a33 ("slab, slub: skip unnecessary kasan_cache_shutdown()")
> Signed-off-by: Shakeel Butt <shakeelb@...gle.com>
> Reported-by: Jason A . Donenfeld <Jason@...c4.com>
> Cc: Christoph Lameter <cl@...ux.com>
> Cc: Pekka Enberg <penberg@...nel.org>
> Cc: David Rientjes <rientjes@...gle.com>
> Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: <stable@...r.kernel.org>
> ---
>  mm/slub.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index a3b8467c14af..731c02b371ae 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3673,9 +3673,23 @@ static void free_partial(struct kmem_cache *s, struct kmem_cache_node *n)
>
>  bool __kmem_cache_empty(struct kmem_cache *s)
>  {
> -       int node;
> +       int cpu, node;
>         struct kmem_cache_node *n;
>
> +       /*
> +        * slabs_node will always be 0 for !CONFIG_SLUB_DEBUG. So, manually
> +        * check slabs for all cpus.
> +        */
> +       if (!IS_ENABLED(CONFIG_SLUB_DEBUG)) {
> +               for_each_online_cpu(cpu) {
> +                       struct kmem_cache_cpu *c;
> +
> +                       c = per_cpu_ptr(s->cpu_slab, cpu);
> +                       if (c->page || slub_percpu_partial(c))
> +                               return false;
> +               }
> +       }
> +
>         for_each_kmem_cache_node(s, node, n)
>                 if (n->nr_partial || slabs_node(s, node))
>                         return false;
> --
> 2.18.0.rc1.244.gcf134e6275-goog
>

I can confirm that this fixes the test case on build.wireguard.com.