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Subject: Re: [PATCH 1/5] mm/debug_pagealloc: clean-up guard page handling code
To: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>, js1304@gmail.com
References: <1470809784-11516-1-git-send-email-iamjoonsoo.kim@lge.com>
 <1470809784-11516-2-git-send-email-iamjoonsoo.kim@lge.com>
 <20160810081453.GB573@swordfish>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Minchan Kim <minchan@kernel.org>, Michal Hocko <mhocko@kernel.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>
From: Vlastimil Babka <vbabka@suse.cz>
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Date: Thu, 11 Aug 2016 11:41:12 +0200
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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.