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

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

[Oscar Salvador]

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.

Signed-off-by: Oscar Salvador <osalvador@techadventures.net>
---
 mm/page_ext.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/page_ext.c b/mm/page_ext.c
index 2c16216c29b6..5295ef331165 100644
--- a/mm/page_ext.c
+++ b/mm/page_ext.c
@@ -59,7 +59,9 @@
  */
 
 static struct page_ext_operations *page_ext_ops[] = {
+#ifdef CONFIG_DEBUG_PAGEALLOC
 	&debug_guardpage_ops,
+#endif
 #ifdef CONFIG_PAGE_OWNER
 	&page_owner_ops,
 #endif
-- 
2.13.5

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

[Andrew Morton]

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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