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Subject: Re: [RFC PATCH 1/1] mm/percpu.c: fix memory leakage issue when
 allocate a odd alignment area
To: Michal Hocko <mhocko@kernel.org>
References: <bc3126cd-226d-91c7-d323-48881095accf@zoho.com>
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 <20161012065332.GA9504@dhcp22.suse.cz>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, zijun_hu@htc.com,
        tj@kernel.org, akpm@linux-foundation.org, cl@linux.com
From: zijun_hu <zijun_hu@zoho.com>
Message-ID: <57FDE531.7060003@zoho.com>
Date: Wed, 12 Oct 2016 15:24:33 +0800
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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@htc.com>
>>>>
>>>> the LSB of a chunk->map element is used for free/in-use flag of a area
>>>> and the other bits for offset, the sufficient and necessary condition of
>>>> this usage is that both size and alignment of a area must be even numbers
>>>> however, pcpu_alloc() doesn't force its @align parameter a even number
>>>> explicitly, so a odd @align maybe causes a series of errors, see below
>>>> example for concrete descriptions.
>>>
>>> Is or was there any user who would use a different than even (or power of 2)
>>> alighment? If not is this really worth handling?
>>>
>>
>> it seems only a power of 2 alignment except 1 can make sure it work very well,
>> that is a strict limit, maybe this more strict limit should be checked
> 
> I fail to see how any other alignment would actually make any sense
> what so ever. Look, I am not a maintainer of this code but adding a new
> code to catch something that doesn't make any sense sounds dubious at
> best to me.
> 
> I could understand this patch if you see a problem and want to prevent
> it from repeating bug doing these kind of changes just in case sounds
> like a bad idea.
> 

thanks for your reply

should we have a generic discussion whether such patches which considers
many boundary or rare conditions are necessary.

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));


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
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