Date: Thu, 13 Oct 2016 19:31:39 -0400
From: Tejun Heo <tj@kernel.org>
To: zijun_hu <zijun_hu@zoho.com>
Cc: akpm@linux-foundation.org, zijun_hu@htc.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, cl@linux.com
Subject: Re: [RFC PATCH 1/1] mm/percpu.c: fix memory leakage issue when
 allocate a odd alignment area
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On Tue, Oct 11, 2016 at 09:24:50PM +0800, 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.
> 
> 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@htc.com>

Nacked-by: Tejun Heo <tj@kernel.org>

This is a fix for an imaginary problem.  The most we should do about
odd alignment is triggering a WARN_ON.

Thanks.

-- 
tejun