Re: [PATCH v3 1/8] mm/page_ext: resurrect struct page extending code for debugging

[Joonsoo Kim]

On Mon, Nov 24, 2014 at 05:15:19PM +0900, Joonsoo Kim wrote:
> When we debug something, we'd like to insert some information to
> every page. For this purpose, we sometimes modify struct page itself.
> But, this has drawbacks. First, it requires re-compile. This makes us
> hesitate to use the powerful debug feature so development process is
> slowed down. And, second, sometimes it is impossible to rebuild the kernel
> due to third party module dependency. At third, system behaviour would be
> largely different after re-compile, because it changes size of struct
> page greatly and this structure is accessed by every part of kernel.
> Keeping this as it is would be better to reproduce errornous situation.
> 
> This feature is intended to overcome above mentioned problems. This feature
> allocates memory for extended data per page in certain place rather than
> the struct page itself. This memory can be accessed by the accessor
> functions provided by this code. During the boot process, it checks whether
> allocation of huge chunk of memory is needed or not. If not, it avoids
> allocating memory at all. With this advantage, we can include this feature
> into the kernel in default and can avoid rebuild and solve related problems.
> 
> Until now, memcg uses this technique. But, now, memcg decides to embed
> their variable to struct page itself and it's code to extend struct page
> has been removed. I'd like to use this code to develop debug feature,
> so this patch resurrect it.
> 
> To help these things to work well, this patch introduces two callbacks
> for clients. One is the need callback which is mandatory if user wants
> to avoid useless memory allocation at boot-time. The other is optional,
> init callback, which is used to do proper initialization after memory
> is allocated. Detailed explanation about purpose of these functions is
> in code comment. Please refer it.
> 
> Others are completely same with previous extension code in memcg.
> 
> v3:
>  minor fix for readable code
> 
> v2:
>  describe overall design at the top of the page extension code.
>  add more description on commit message.
> 
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

Hello, Andrew.

Could you fold following fix into the merged patch?
It fixes the problem on !CONFIG_SPARSEMEM which is reported by
0day kernel testing robot.

https://lkml.org/lkml/2014/11/28/123

Thanks.


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