Re: [PATCH] mm/page_ext.c: Make page_ext_init a noop when CONFIG_PAGE_EXTENSION but nothing uses it

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 5 Jan 2018 14:02:35 +0100 Oscar Salvador <osalvador@techadventures.net> wrote:

> static struct page_ext_operations *page_ext_ops[] always contains debug_guardpage_ops,
> 
> static struct page_ext_operations *page_ext_ops[] = {
>         &debug_guardpage_ops,
>  #ifdef CONFIG_PAGE_OWNER
>         &page_owner_ops,
>  #endif
> ...
> }
> 
> but for it to work, CONFIG_DEBUG_PAGEALLOC must be enabled first.
> If someone has CONFIG_PAGE_EXTENSION, but has none of its users,
> eg: (CONFIG_PAGE_OWNER, CONFIG_DEBUG_PAGEALLOC, CONFIG_IDLE_PAGE_TRACKING), we can shrink page_ext_init()
> to a simple retq.
> 
> $ size vmlinux  (before patch)
>    text	   data	    bss	    dec	    hex	filename
> 14356698	5681582	1687748	21726028	14b834c	vmlinux
> 
> $ size vmlinux  (after patch)
>    text	   data	    bss	    dec	    hex	filename
> 14356008	5681538	1687748	21725294	14b806e	vmlinux
> 
> On the other hand, it might does not even make sense, since if someone
> enables CONFIG_PAGE_EXTENSION, I would expect him to enable also at least
> one of its users, but I wanted to see what you guys think.

Presumably the CONFIG_PAGE_EXTENSION users should `select'
CONFIG_PAGE_EXTENSION so the situation doesn't arise.

(or does it?  I have a vague memory that if CONFIG_A selects CONFIG_B
and you then set CONFIG_A=n, CONFIG_B remains enabled?)

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