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

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Date:   Tue, 14 Jun 2022 13:47:35 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     tury <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 11:26, tury wrote:
> 
> 
> 在 2022年06月14日 16:48, Vlastimil Babka 写道:
>> 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.
> Thanks for the advice ,I'll be re-patching

However that was meant just for your information/learning, the patch is
still unecessary as I wrote above, so I will not merge it so we don't
complicate the code needlessly.

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