linux-kernel - Re: [RFC PATCH 1/1] mm/percpu.c: fix memory leakage issue when allocate a odd alignment area

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Date:   Wed, 12 Oct 2016 16:44:31 +0800
From:   zijun_hu <zijun_hu@...o.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org, zijun_hu@....com,
        tj@...nel.org, akpm@...ux-foundation.org, cl@...ux.com
Subject: Re: [RFC PATCH 1/1] mm/percpu.c: fix memory leakage issue when
 allocate a odd alignment area

On 10/12/2016 04:25 PM, Michal Hocko wrote:
> On Wed 12-10-16 15:24:33, zijun_hu wrote:
>> On 10/12/2016 02:53 PM, Michal Hocko wrote:
>>> On Wed 12-10-16 08:28:17, zijun_hu wrote:
>>>> On 2016/10/12 1:22, Michal Hocko wrote:
>>>>> On Tue 11-10-16 21:24:50, zijun_hu wrote:
>>>>>> From: zijun_hu <zijun_hu@....com>
>>>>>>
>> should we have a generic discussion whether such patches which considers
>> many boundary or rare conditions are necessary.
> 
> In general, I believe that kernel internal interfaces which have no
> userspace exposure shouldn't be cluttered with sanity checks.
> 

you are right and i agree with you. but there are many internal interfaces
perform sanity checks in current linux sources

>> i found the following code segments in mm/vmalloc.c
>> static struct vmap_area *alloc_vmap_area(unsigned long size,
>>                                 unsigned long align,
>>                                 unsigned long vstart, unsigned long vend,
>>                                 int node, gfp_t gfp_mask)
>> {
>> ...
>>
>>         BUG_ON(!size);
>>         BUG_ON(offset_in_page(size));
>>         BUG_ON(!is_power_of_2(align));
> 
> See a recent Linus rant about BUG_ONs. These BUG_ONs are quite old and
> from a quick look they are even unnecessary. So rather than adding more
> of those, I think removing those that are not needed is much more
> preferred.
>
i notice that, and the above code segments is used to illustrate that
input parameter checking is necessary sometimes

>> should we make below declarations as conventions
>> 1) when we say 'alignment', it means align to a power of 2 value
>>    for example, aligning value @v to @b implicit @v is power of 2
>>    , align 10 to 4 is 12
> 
> alignment other than power-of-two makes only very limited sense to me.
> 
you are right and i agree with you.
>> 2) when we say 'round value @v up/down to boundary @b', it means the 
>>    result is a times of @b,  it don't requires @b is a power of 2
> 

i will write to linus to ask for opinions whether we should declare 
the meaning of 'align' and 'round up/down' formally and whether such
patches are necessary