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:   Fri, 14 Oct 2016 08:23:06 +0800
From:   zijun_hu <zijun_hu@...o.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     zijun_hu@....com, linux-mm@...ck.org, linux-kernel@...r.kernel.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 2016/10/14 7:31, Tejun Heo wrote:
> On Tue, Oct 11, 2016 at 09:24:50PM +0800, 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.
>>
>> lets assume area [16, 36) is free but its previous one is in-use, we want
>> to allocate a @size == 8 and @align == 7 area. the larger area [16, 36) is
>> split to three areas [16, 21), [21, 29), [29, 36) eventually. however, due
>> to the usage for a chunk->map element, the actual offset of the aim area
>> [21, 29) is 21 but is recorded in relevant element as 20; moreover the
>> residual tail free area [29, 36) is mistook as in-use and is lost silently
>>
>> as explained above, inaccurate either offset or free/in-use state of
>> a area is recorded into relevant chunk->map element if request a odd
>> alignment area, and so causes memory leakage issue
>>
>> fix it by forcing the @align of a area to allocate a even number
>> as do for @size.
>>
>> BTW, macro ALIGN() within pcpu_fit_in_area() is replaced by roundup() too
>> due to back reason. in order to align a value @v up to @a boundary, macro
>> roundup(v, a) is more generic than ALIGN(x, a); the latter doesn't work
>> well when @a isn't a power of 2 value. for example, roundup(10, 6) == 12
>> but ALIGN(10, 6) == 10, the former result is desired obviously
>>
>> Signed-off-by: zijun_hu <zijun_hu@....com>
> 
> Nacked-by: Tejun Heo <tj@...nel.org>
> 
> This is a fix for an imaginary problem.  The most we should do about
> odd alignment is triggering a WARN_ON.
> 
for the current code, only power of 2 alignment value can works well

is it acceptable to performing a power of 2 checking and returning error code
if fail?