Re: [PATCH 2/2] mm: compaction: allow isolation of lower order buddy pages

[David Rientjes <rientjes@google.com>]

On Thu, 5 Apr 2012, Bartlomiej Zolnierkiewicz wrote:

> diff --git a/mm/compaction.c b/mm/compaction.c
> index bc77135..642c17a 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -115,8 +115,8 @@ static bool suitable_migration_target(struct page *page)
>  	if (migratetype == MIGRATE_ISOLATE || migratetype == MIGRATE_RESERVE)
>  		return false;
>  
> -	/* If the page is a large free page, then allow migration */
> -	if (PageBuddy(page) && page_order(page) >= pageblock_order)
> +	/* If the page is a free page, then allow migration */
> +	if (PageBuddy(page))
>  		return true;
>  
>  	/* If the block is MIGRATE_MOVABLE, allow migration */

So when we try to allocate a 2M hugepage through the buddy allocator where 
the pageblock is also 2M, wouldn't this result in a lot of unnecessary 
migration of memory that may not end up defragmented enough for the 
allocation to succeed?  Sounds like a regression for hugepage allocation.

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