Re: [PATCH v2] mm: remove MIGRATE_ISOLATE check in hotpath

[Andrew Morton]

On Tue, 15 Jan 2013 09:16:46 +0900
Minchan Kim <minchan@xxxxxxxxxx> wrote:

> Now mm several functions test MIGRATE_ISOLATE and some of those
> are hotpath but MIGRATE_ISOLATE is used only if we enable
> CONFIG_MEMORY_ISOLATION(ie, CMA, memory-hotplug and memory-failure)
> which are not common config option. So let's not add unnecessary
> overhead and code when we don't enable CONFIG_MEMORY_ISOLATION.

ugh.  Better than nothing, I guess.

There remain call sites which do open-coded

	get_pageblock_migratetype(page) != MIGRATE_ISOLATE

(undo_isolate_page_range() is one).  Wanna clean these up as well?

>
> ...
>
> @@ -683,7 +683,7 @@ static void free_one_page(struct zone *zone, struct page *page, int order,
>  	zone->pages_scanned = 0;
>  
>  	__free_one_page(page, zone, order, migratetype);
> -	if (unlikely(migratetype != MIGRATE_ISOLATE))
> +	if (unlikely(!is_migrate_isolate(migratetype)))
>  		__mod_zone_freepage_state(zone, 1 << order, migratetype);
>  	spin_unlock(&zone->lock);
>  }

The code both before and after this patch is assuming that the
migratetype in free_one_page is likely to be MIGRATE_ISOLATE.  Seems
wrong.  If CONFIG_MEMORY_ISOLATION=n this ends up doing
if(unlikely(true)) which is harmless-but-amusing.

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