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 08:28:17 +0800
From:   zijun_hu <zijun_hu@...o.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     zijun_hu@....com, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        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 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>
>>
>> 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 don't know since there are too many sources and too many users and too many
use cases. even if nobody, i can't be sure that it doesn't happens in the future

it is worth since below reasons
1) if it is used in right ways, this patch have no impact; otherwise, it can alert
   user by warning message and correct the behavior.
   is it better that a warning message and correcting than resulting in many terrible
   error silently under a special case by change?
   it can make program more stronger.

2) does any alignment but 1 means a power of 2 alignment conventionally and implicitly? 
   if not, is it better that adjusting both @align and @size uniformly based on the sufficient
   necessary condition than mixing supposing one part is right and correcting the other?
   i find that there is BUG_ON(!is_power_of_2(align)) statement in mm/vmalloc.c

3) this simple fix can make the function applicable in wider range, it hints the reader
   that the lowest requirement for alignment is a even number

4) for char a[10][10]; char (*p)[10]; if a user want to allocate a @size = 10 and
   @align = 10 memory block, should we reject the user's request?