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

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Date:	Fri, 12 Aug 2016 21:25:37 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	js1304@...il.com, Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan@...nel.org>,
	Michal Hocko <mhocko@...nel.org>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH 1/5] mm/debug_pagealloc: clean-up guard page handling code

On (08/11/16 11:41), Vlastimil Babka wrote:
> 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.

ah, indeed. didn't notice it.

	-ss