Re: [PATCH] mm: Improve documentation of page_order

[Laura Abbott <lauraa@codeaurora.org>]

On 1/17/2014 6:32 AM, 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@suse.de>
> ---
>   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
                                   page_zone(page)->lock here?
> + * the page cannot be allocated or merged in parallel.
>    */
>   static inline unsigned long page_order(struct page *page)
>   {
>
> --
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