Re: [PATCH 1/5] mm/compaction: disallow high-order page formigration target

[Mel Gorman]

On Fri, Feb 07, 2014 at 02:08:42PM +0900, Joonsoo Kim wrote:
> Purpose of compaction is to get a high order page. Currently, if we find
> high-order page while searching migration target page, we break it to
> order-0 pages and use them as migration target. It is contrary to purpose
> of compaction, so disallow high-order page to be used for
> migration target.
> 
> Additionally, clean-up logic in suitable_migration_target() to simply.
> There is no functional changes from this clean-up.
> 
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 3a91a2e..bbe1260 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -217,21 +217,12 @@ static inline bool compact_trylock_irqsave(spinlock_t *lock,
>  /* Returns true if the page is within a block suitable for migration to */
>  static bool suitable_migration_target(struct page *page)
>  {
> -	int migratetype = get_pageblock_migratetype(page);
> -
> -	/* Don't interfere with memory hot-remove or the min_free_kbytes blocks */
> -	if (migratetype == MIGRATE_RESERVE)
> -		return false;
> -

Why is this check removed? The reservation blocks are preserved as
short-lived high-order atomic allocations depend on them.

> -	if (is_migrate_isolate(migratetype))
> -		return false;
> -

Why is this check removed?

> -	/* If the page is a large free page, then allow migration */
> +	/* If the page is a large free page, then disallow migration */
>  	if (PageBuddy(page) && page_order(page) >= pageblock_order)
> -		return true;
> +		return false;
>  

The reason why this was originally allowed was to allow pageblocks that were
marked MIGRATE_UNMOVABLE or MIGRATE_RECLAIMABLE to be used as compaction
targets. However, compaction should not even be running if this is the
case so the change makes sense.

>  	/* If the block is MIGRATE_MOVABLE or MIGRATE_CMA, allow migration */
> -	if (migrate_async_suitable(migratetype))
> +	if (migrate_async_suitable(get_pageblock_migratetype(page)))
>  		return true;
>  
>  	/* Otherwise skip the block */
> -- 
> 1.7.9.5
> 

-- 
Mel Gorman
SUSE Labs
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