Re: [PATCH 1/5] mm/debug_pagealloc: clean-up guard page handling code

[Vlastimil Babka]

On 08/10/2016 10:14 AM, Sergey Senozhatsky wrote:
> > @@ -1650,18 +1655,15 @@ static inline void expand(struct zone *zone, struct page *page,
> >  		size >>= 1;
> >  		VM_BUG_ON_PAGE(bad_range(zone, &page[size]), &page[size]);
> >
> > -		if (IS_ENABLED(CONFIG_DEBUG_PAGEALLOC) &&
> > -			debug_guardpage_enabled() &&
> > -			high < debug_guardpage_minorder()) {
> > -			/*
> > -			 * Mark as guard pages (or page), that will allow to
> > -			 * merge back to allocator when buddy will be freed.
> > -			 * Corresponding page table entries will not be touched,
> > -			 * pages will stay not present in virtual address space
> > -			 */
> > -			set_page_guard(zone, &page[size], high, migratetype);
> > +		/*
> > +		 * Mark as guard pages (or page), that will allow to
> > +		 * merge back to allocator when buddy will be freed.
> > +		 * Corresponding page table entries will not be touched,
> > +		 * pages will stay not present in virtual address space
> > +		 */
> > +		if (set_page_guard(zone, &page[size], high, migratetype))
> >  			continue;
> > -		}
>
> so previously IS_ENABLED(CONFIG_DEBUG_PAGEALLOC) could have optimized out
> the entire branch -- no set_page_guard() invocation and checks, right? but
> now we would call set_page_guard() every time?

No, there's a !CONFIG_DEBUG_PAGEALLOC version of set_page_guard() that 
returns false (static inline), so this whole if will be eliminated by 
the compiler, same as before.