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

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