linux-kernel - Re: [PATCH] mm/hotplug: fix a drain pcp bug when offline pages

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Date:	Thu, 01 Aug 2013 11:59:12 -0700
From:	Cody P Schafer <cody@...ux.vnet.ibm.com>
To:	Xishi Qiu <qiuxishi@...wei.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Liujiang <jiang.liu@...wei.com>,
	Minchan Kim <minchan@...nel.org>,
	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	b.zolnierkie@...sung.com, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/hotplug: fix a drain pcp bug when offline pages

On 08/01/2013 02:18 AM, Xishi Qiu wrote:
> __offline_pages()
>     start_isolate_page_range()
>        set_migratetype_isolate()
>           set_pageblock_migratetype() -> this pageblock will be marked as MIGRATE_ISOLATE
>           move_freepages_block() -> pages in PageBuddy will be moved into MIGRATE_ISOLATE list
>           drain_all_pages() -> drain PCP
>              free_pcppages_bulk()
>                 mt = get_freepage_migratetype(page); -> PCP's migratetype is not MIGRATE_ISOLATE
>                 __free_one_page(page, zone, 0, mt); -> so PCP will not be freed into into MIGRATE_ISOLATE list
>
> In this case, the PCP may be allocated again, because they are not in
> PageBuddy's MIGRATE_ISOLATE list. This will cause offline_pages failed.
>
> Signed-off-by: Xishi Qiu <qiuxishi@...wei.com>
> ---
>   mm/page_alloc.c     |   10 ++++++----
>   mm/page_isolation.c |   15 ++++++++++++++-
>   2 files changed, 20 insertions(+), 5 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index b100255..d873471 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -965,11 +965,13 @@ int move_freepages(struct zone *zone,
>   		}
>
>   		order = page_order(page);
> -		list_move(&page->lru,
> -			  &zone->free_area[order].free_list[migratetype]);
> -		set_freepage_migratetype(page, migratetype);
> +		if (get_freepage_migratetype(page) != migratetype) {
> +			list_move(&page->lru,
> +				&zone->free_area[order].free_list[migratetype]);
> +			set_freepage_migratetype(page, migratetype);
> +			pages_moved += 1 << order;
> +		}
>   		page += 1 << order;
> -		pages_moved += 1 << order;

So this looks like it changes the return from move_freepages() to be the 
"pages moved" from "the pages now belonging to the passed migrate type".

The user of move_freepages_block()'s return value (and thus the return 
value of move_freepages()) in mm/page_alloc.c expects that it is the 
original meaning. The users in page_isolation.c expect it is the new 
meaning. Those need to be reconciled.

>   	}
>
>   	return pages_moved;
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index 383bdbb..ba1afc9 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -65,8 +65,21 @@ out:
>   	}
>
>   	spin_unlock_irqrestore(&zone->lock, flags);
> -	if (!ret)
> +
> +	if (!ret) {
>   		drain_all_pages();
> +		/*
> +		 * When drain_all_pages() frees cached pages into the buddy
> +		 * system, it uses the stale migratetype cached in the
> +		 * page->index field, so try to move free pages to ISOLATE
> +		 * list again.
> +		 */
> +		spin_lock_irqsave(&zone->lock, flags);
> +		nr_pages = move_freepages_block(zone, page, MIGRATE_ISOLATE);
> +		__mod_zone_freepage_state(zone, -nr_pages, migratetype);
> +		spin_unlock_irqrestore(&zone->lock, flags);
> +	}
> +

Could we teach drain_all_pages() to use the right migrate type instead 
(or add something similar that does)? (pages could be reallocated 
between the drain_all_pages() and move_freepages_block()).

>   	return ret;
>   }
>

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