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

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  Re: [RFC PATCH 1/1] mm/percpu.c: fix memory leakage issue when  allocate a odd alignment area Tejun Heo <tj@kernel.org> - 2016-10-14 01:40 +0200
    Re: [RFC PATCH 1/1] mm/percpu.c: fix memory leakage issue when  allocate a odd alignment area zijun_hu <zijun_hu@zoho.com> - 2016-10-14 02:30 +0200
      Re: [RFC PATCH 1/1] mm/percpu.c: fix memory leakage issue when  allocate a odd alignment area Tejun Heo <tj@kernel.org> - 2016-10-14 02:50 +0200
        Re: [RFC PATCH 1/1] mm/percpu.c: fix memory leakage issue when  allocate a odd alignment area zijun_hu <zijun_hu@zoho.com> - 2016-10-14 03:00 +0200

#1500624 — Re: [RFC PATCH 1/1] mm/percpu.c: fix memory leakage issue when allocate a odd alignment area

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

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#1500646

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@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.
> 
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?

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#1500650

Hello,

On Fri, Oct 14, 2016 at 08:23:06AM +0800, zijun_hu wrote:
> 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?

Yeah, just add is_power_of_2() test to the existing sanity check.

Thanks.

-- 
tejun

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#1500654

On 2016/10/14 8:28, Tejun Heo wrote:
> Hello,
> 
> On Fri, Oct 14, 2016 at 08:23:06AM +0800, zijun_hu wrote:
>> 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?
> 
> Yeah, just add is_power_of_2() test to the existing sanity check.
> 
> Thanks.
> 
okay. i will do that

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