Re: [PATCH 03/10] mm/page_alloc: handle page on pcp correctly if it's pageblock is isolated

[Vlastimil Babka]

On 07/04/2014 09:57 AM, Joonsoo Kim wrote:
> If pageblock of page on pcp are isolated now, we should free it to isolate
> buddy list to prevent future allocation on it. But current code doesn't
> do this.
>
> Moreover, there is a freepage counting problem on current code. Although
> pageblock of page on pcp are isolated now, it could go normal buddy list,
> because get_onpcp_migratetype() will return non-isolate migratetype.

get_onpcp_migratetype() is only introduced in later patch.

> In this case, we should do either adding freepage count or changing
> migratetype to MIGRATE_ISOLATE, but, current code do neither.

I wouldn't say it "do neither". It already limits the freepage counting 
to !MIGRATE_ISOLATE case (and it's not converted to 
__mod_zone_freepage_state for some reason). So there's accounting 
mismatch in addition to buddy list misplacement.

> This patch fixes these two problems by handling pageblock migratetype
> before calling __free_one_page(). And, if we find the page on isolated
> pageblock, change migratetype to MIGRATE_ISOLATE to prevent future
> allocation of this page and freepage counting problem.

So although this is not an addition of a new pageblock migratetype check 
to the fast path (the check is already there), I would prefer removing 
the check :) With the approach of pcplists draining outlined in my reply 
to 00/10, we would allow a misplacement to happen (and the page 
accounted as freepage) immediately followed by move_frepages_block which 
would place the page onto isolate freelist with the rest. Anything newly 
freed will get isolate_migratetype determined in free_hot_cold_page or 
__free_pages_ok (where it would need moving the migratepage check under 
the disabled irq part) and be placed and buddy-merged properly.

> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> ---
>   mm/page_alloc.c |   14 ++++++++------
>   1 file changed, 8 insertions(+), 6 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index aeb51d1..99c05f7 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -719,15 +719,17 @@ static void free_pcppages_bulk(struct zone *zone, int count,
>   			page = list_entry(list->prev, struct page, lru);
>   			/* must delete as __free_one_page list manipulates */
>   			list_del(&page->lru);
> -			mt = get_freepage_migratetype(page);
> +
> +			if (unlikely(is_migrate_isolate_page(page))) {
> +				mt = MIGRATE_ISOLATE;
> +			} else {
> +				mt = get_freepage_migratetype(page);
> +				__mod_zone_freepage_state(zone, 1, mt);
> +			}
> +
>   			/* MIGRATE_MOVABLE list may include MIGRATE_RESERVEs */
>   			__free_one_page(page, page_to_pfn(page), zone, 0, mt);
>   			trace_mm_page_pcpu_drain(page, 0, mt);
> -			if (likely(!is_migrate_isolate_page(page))) {
> -				__mod_zone_page_state(zone, NR_FREE_PAGES, 1);
> -				if (is_migrate_cma(mt))
> -					__mod_zone_page_state(zone, NR_FREE_CMA_PAGES, 1);
> -			}
>   		} while (--to_free && --batch_free && !list_empty(list));
>   	}
>   	spin_unlock(&zone->lock);

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