Re: [PATCH v2 2/8] mm/page_alloc: correct to clear guard attribute in DEBUG_PAGEALLOC

[Minchan Kim]

On Wed, Aug 06, 2014 at 04:18:30PM +0900, Joonsoo Kim wrote:
> In __free_one_page(), we check the buddy page if it is guard page.
> And, if so, we should clear guard attribute on the buddy page. But,
> currently, we clear original page's order rather than buddy one's.
> This doesn't have any problem, because resetting buddy's order
> is useless and the original page's order is re-assigned soon.
> But, it is better to correct code.
> 
> Additionally, I change (set/clear)_page_guard_flag() to
> (set/clear)_page_guard() and makes these functions do all works
> needed for guard page. This may make code more understandable.
> 
> One more thing, I did in this patch, is that fixing freepage accounting.
> If we clear guard page and link it onto isolate buddy list, we should
> not increase freepage count.

You are saying just "shouldn't do that" but don't say "why" and "result"
I know the reason but as you know, I'm one of the person who is rather
familiar with this part but I guess others should spend some time to get.
Kind detail description is never to look down on person. :)

> 

Nice catch, Joonsoo! But what make me worry is is this patch makes 3 thing
all at once.

1. fix - no candidate for stable
2. clean up
3. fix - candidate for stable.

Could you separate 3 and (1,2) in next spin?


> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> ---
>  mm/page_alloc.c |   29 ++++++++++++++++-------------
>  1 file changed, 16 insertions(+), 13 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 44672dc..3e1e344 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -441,18 +441,28 @@ static int __init debug_guardpage_minorder_setup(char *buf)
>  }
>  __setup("debug_guardpage_minorder=", debug_guardpage_minorder_setup);
>  
> -static inline void set_page_guard_flag(struct page *page)
> +static inline void set_page_guard(struct zone *zone, struct page *page,
> +				unsigned int order, int migratetype)
>  {
>  	__set_bit(PAGE_DEBUG_FLAG_GUARD, &page->debug_flags);
> +	set_page_private(page, order);
> +	/* Guard pages are not available for any usage */
> +	__mod_zone_freepage_state(zone, -(1 << order), migratetype);
>  }
>  
> -static inline void clear_page_guard_flag(struct page *page)
> +static inline void clear_page_guard(struct zone *zone, struct page *page,
> +				unsigned int order, int migratetype)
>  {
>  	__clear_bit(PAGE_DEBUG_FLAG_GUARD, &page->debug_flags);
> +	set_page_private(page, 0);
> +	if (!is_migrate_isolate(migratetype))
> +		__mod_zone_freepage_state(zone, (1 << order), migratetype);
>  }
>  #else
> -static inline void set_page_guard_flag(struct page *page) { }
> -static inline void clear_page_guard_flag(struct page *page) { }
> +static inline void set_page_guard(struct zone *zone, struct page *page,
> +				unsigned int order, int migratetype) {}
> +static inline void clear_page_guard(struct zone *zone, struct page *page,
> +				unsigned int order, int migratetype) {}
>  #endif
>  
>  static inline void set_page_order(struct page *page, unsigned int order)
> @@ -594,10 +604,7 @@ static inline void __free_one_page(struct page *page,
>  		 * merge with it and move up one order.
>  		 */
>  		if (page_is_guard(buddy)) {
> -			clear_page_guard_flag(buddy);
> -			set_page_private(page, 0);
> -			__mod_zone_freepage_state(zone, 1 << order,
> -						  migratetype);
> +			clear_page_guard(zone, buddy, order, migratetype);
>  		} else {
>  			list_del(&buddy->lru);
>  			zone->free_area[order].nr_free--;
> @@ -876,11 +883,7 @@ static inline void expand(struct zone *zone, struct page *page,
>  			 * pages will stay not present in virtual address space
>  			 */
>  			INIT_LIST_HEAD(&page[size].lru);
> -			set_page_guard_flag(&page[size]);
> -			set_page_private(&page[size], high);
> -			/* Guard pages are not available for any usage */
> -			__mod_zone_freepage_state(zone, -(1 << high),
> -						  migratetype);
> +			set_page_guard(zone, &page[size], high, migratetype);
>  			continue;
>  		}
>  #endif
> -- 
> 1.7.9.5
> 
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