Re: [PATCH] mm: Improve documentation of page_order

[Rafael Aquini]

On Fri, Jan 17, 2014 at 02:32:21PM +0000, Mel Gorman wrote:
> Developers occasionally try and optimise PFN scanners by using page_order
> but miss that in general it requires zone->lock. This has happened twice for
> compaction.c and rejected both times.  This patch clarifies the documentation
> of page_order and adds a note to compaction.c why page_order is not used.
> 
> Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
> ---
>  mm/compaction.c | 5 ++++-
>  mm/internal.h   | 8 +++++---
>  2 files changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index f58bcd0..f91d26b 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -522,7 +522,10 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
>  		if (!isolation_suitable(cc, page))
>  			goto next_pageblock;
>  
> -		/* Skip if free */
> +		/*
> +		 * Skip if free. page_order cannot be used without zone->lock
> +		 * as nothing prevents parallel allocations or buddy merging.
> +		 */
>  		if (PageBuddy(page))
>  			continue;
>  
> diff --git a/mm/internal.h b/mm/internal.h
> index 684f7aa..09cd8be 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -144,9 +144,11 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
>  #endif
>  
>  /*
> - * function for dealing with page's order in buddy system.
> - * zone->lock is already acquired when we use these.
> - * So, we don't need atomic page->flags operations here.
> + * This functions returns the order of a free page in the buddy system.
> + * In general, page_zone(page)->lock must be held by the caller to prevent
> + * the page being allocated in parallel and returning garbage as the order.
> + * If the caller does not hold page_zone(page), they must guarantee that
> + * the page cannot be allocated or merged in parallel.
>   */
>  static inline unsigned long page_order(struct page *page)
>  {

Acked-by: Rafael Aquini <aquini@xxxxxxxxxx>


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