Re: [PATCH v2 5/6] mm/page_ext: support extra space allocation by page_ext user

[Vlastimil Babka]

On 08/16/2016 04:51 AM, js1304@xxxxxxxxx wrote:
> From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
>
> Until now, if some page_ext users want to use it's own field on page_ext,
> it should be defined in struct page_ext by hard-coding. It has a problem
> that wastes memory in following situation.
>
> struct page_ext {
>  #ifdef CONFIG_A
> 	int a;
>  #endif
>  #ifdef CONFIG_B
> 	int b;
>  #endif
> };
>
> Assume that kernel is built with both CONFIG_A and CONFIG_B.
> Even if we enable feature A and doesn't enable feature B at runtime,
> each entry of struct page_ext takes two int rather than one int.
> It's undesirable result so this patch tries to fix it.
>
> To solve above problem, this patch implements to support extra space
> allocation at runtime. When need() callback returns true, it's extra
> memory requirement is summed to entry size of page_ext. Also, offset
> for each user's extra memory space is returned. With this offset,
> user can use this extra space and there is no need to define needed
> field on page_ext by hard-coding.
>
> This patch only implements an infrastructure. Following patch will use it
> for page_owner which is only user having it's own fields on page_ext.
>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>