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

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Message-ID: <172b4c63-b519-cf1d-ed68-1f85f2caed14@suse.cz>
Date:	Thu, 11 Aug 2016 11:41:12 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>, js1304@...il.com
Cc:	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 08/10/2016 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.