Re: [PATCH 08/15] mm: mmzone: MIGRATE_CMA migration type added

Michal Nazarewicz
2012-01-30T13:06:53Z

> On Thu, Jan 26, 2012 at 10:00:50AM +0100, Marek Szyprowski wrote:
>> From: Michal Nazarewicz <mina86@mina86.com>
>> @@ -875,10 +895,15 @@ struct page *__rmqueue_smallest(struct zone *zone, unsigned int order,
>> * This array describes the order lists are fallen back to when
>> * the free lists for the desirable migrate type are depleted
>> */
>> -static int fallbacks[MIGRATE_TYPES][3] = {
>> +static int fallbacks[MIGRATE_TYPES][4] = {
>> [MIGRATE_UNMOVABLE] = { MIGRATE_RECLAIMABLE, MIGRATE_MOVABLE, MIGRATE_RESERVE },
>> [MIGRATE_RECLAIMABLE] = { MIGRATE_UNMOVABLE, MIGRATE_MOVABLE, MIGRATE_RESERVE },
>> +#ifdef CONFIG_CMA
>> + [MIGRATE_MOVABLE] = { MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE, MIGRATE_CMA , MIGRATE_RESERVE },

On Mon, 30 Jan 2012 13:35:42 +0100, Mel Gorman <mel@csn.ul.ie> wrote:
> This is a curious choice. MIGRATE_CMA is allowed to contain movable
> pages. By using MIGRATE_RECLAIMABLE and MIGRATE_UNMOVABLE for movable
> pages instead of MIGRATE_CMA, you increase the changes that unmovable
> pages will need to use MIGRATE_MOVABLE in the future which impacts
> fragmentation avoidance. I would recommend that you change this to
>
> { MIGRATE_CMA, MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE, MIGRATE_RESERVE }

At the beginning the idea was to try hard not to get pages from MIGRATE_CMA
allocated at all, thus it was put at the end of the fallbacks list, but on
a busy system this probably won't help anyway, so I'll change it per your
suggestion.

>> @@ -1017,11 +1049,14 @@ __rmqueue_fallback(struct zone *zone, int order, int start_migratetype)
>> rmv_page_order(page);
>>
>> /* Take ownership for orders >= pageblock_order */
>> - if (current_order >= pageblock_order)
>> + if (current_order >= pageblock_order &&
>> + !is_pageblock_cma(page))
>> change_pageblock_range(page, current_order,
>> start_migratetype);
>>
>> - expand(zone, page, order, current_order, area, migratetype);
>> + expand(zone, page, order, current_order, area,
>> + is_migrate_cma(start_migratetype)
>> + ? start_migratetype : migratetype);
>>
>
> What is this check meant to be doing?
>
> start_migratetype is determined by allocflags_to_migratetype() and
> that never will be MIGRATE_CMA so is_migrate_cma(start_migratetype)
> should always be false.

Right, thanks! This should be the other way around, ie.:

+ expand(zone, page, order, current_order, area,
+ is_migrate_cma(migratetype)
+ ? migratetype : start_migratetype);

I'll fix this and the calls to is_pageblock_cma().

--
Best regards, _ _
.o. | Liege of Serenely Enlightened Majesty of o' \,=./ `o
..o | Computer Science, Michał “mina86” Nazarewicz (o o)
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