linux-kernel - Re: [PATCH] mm: check the function kmalloc

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Date:   Tue, 14 Jun 2022 10:48:42 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Ren Yu <renyu@...china.com>, cl@...ux.com
Cc:     penberg@...nel.org, rientjes@...gle.com, iamjoonsoo.kim@....com,
        akpm@...ux-foundation.org, roman.gushchin@...ux.dev,
        42.hyeyoo@...il.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, liqiong@...china.com,
        qixu@...china.com, hukun@...china.com, yuzhe@...china.com
Subject: Re: [PATCH] mm: check the function kmalloc_slab return value

On 6/14/22 10:39, Ren Yu wrote:
> As the possible failure of the kmalloc_slab,
> it should be better to check it.

AFAIK failure is not possible, kmalloc_slab() is not an allocation function,
it just returns a member of kmalloc_caches array, which is initialized
elsewhere and shouldn't contain NULLs. So the patch seems unnecessary to me.

> Signed-off-by: Ren Yu <renyu@...china.com>
> Reported-by: kernel test robot <lkp@...el.com>
> ---
> v2:
> - fix build waring integer from pointer without a cast
> ---
> ---
>  mm/slab.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/slab.c b/mm/slab.c
> index f8cd00f4ba13..72135e555827 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -2064,6 +2064,8 @@ int __kmem_cache_create(struct kmem_cache *cachep, slab_flags_t flags)
>  	if (OFF_SLAB(cachep)) {
>  		cachep->freelist_cache =
>  			kmalloc_slab(cachep->freelist_size, 0u);
> +		if (unlikely(ZERO_OR_NULL_PTR(cachep->freelist_cache)))

The usual way is "if (!cachep->freelist_cache)". Not sure why check for ZERO.

> +			return cachep->freelist_cache;

So in case of NULL this would return NULL, thus 0, but __kmem_cache_create()
return 0 on success, so it's wrong. You would have to return e.g. -ENOMEM.

>  	}
>  
>  	err = setup_cpu_cache(cachep, gfp);