Date: Wed, 22 Aug 2012 12:34:41 +0900
From: Minchan Kim <minchan@kernel.org>
To: qiuxishi <qiuxishi@gmail.com>
Cc: akpm@linux-foundation.org, lliubbo@gmail.com, jiang.liu@huawei.com,
        mgorman@suse.de, kamezawa.hiroyu@jp.fujitsu.com, mhocko@suse.cz,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org, qiuxishi@huawei.com,
        wujianguo@huawei.com, bessel.wang@huawei.com, guohanjun@huawei.com,
        chenkeping@huawei.com, yinghai@kernel.org, wency@cn.fujitsu.com
Subject: Re: [PATCH] memory-hotplug: fix a drain pcp bug when offline pages
Message-ID: <20120822033441.GB24667@bbox>
References: <50337B15.2090701@gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <50337B15.2090701@gmail.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2905
Lines: 76

Hello Xishi,

On Tue, Aug 21, 2012 at 08:12:05PM +0800, qiuxishi wrote:
> From: Xishi Qiu <qiuxishi@huawei.com>
> 
> When offline a section, we move all the free pages and pcp into MIGRATE_ISOLATE list first.
> start_isolate_page_range()
> 	set_migratetype_isolate()
> 		drain_all_pages(),
> 
> Here is a problem, it is not sure that pcp will be moved into MIGRATE_ISOLATE list. They may
> be moved into MIGRATE_MOVABLE list because page_private() maybe 2. So when finish migrating
> pages, the free pages from pcp may be allocated again, and faild in check_pages_isolated().
> drain_all_pages()
> 	drain_local_pages()
> 		drain_pages()
> 			free_pcppages_bulk()
> 				__free_one_page(page, zone, 0, page_private(page));
> 
> If we add move_freepages_block() after drain_all_pages(), it can not sure that all the pcp
> will be moved into MIGRATE_ISOLATE list when the system works on high load. The free pages
> which from pcp may immediately be allocated again.
> 
> I think the similar bug described in http://marc.info/?t=134250882300003&r=1&w=2

Yes. I reported the problem a few month ago but it's not real bug in practice
but found by my eyes during looking the code so I wanted to confirm the problem.

Do you find that problem in real practice? or just code review?

Anyway, I don't like your approach which I already considered because it hurts hotpath
while the race is really unlikely. Get_pageblock_migratetype is never trivial.
We should avoid the overhead in hotpath and move into memory-hotplug itself.
Do you see my patch in https://patchwork.kernel.org/patch/1225081/ ?

> 
> 
> Signed-off-by: Xishi Qiu <qiuxishi@huawei.com>
> ---
>  mm/page_alloc.c |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index d0723b2..501f6de 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -673,7 +673,8 @@ static void free_pcppages_bulk(struct zone *zone, int count,
>  			/* must delete as __free_one_page list manipulates */
>  			list_del(&page->lru);
>  			/* MIGRATE_MOVABLE list may include MIGRATE_RESERVEs */
> -			__free_one_page(page, zone, 0, page_private(page));
> +			__free_one_page(page, zone, 0,
> +					get_pageblock_migratetype(page));
>  			trace_mm_page_pcpu_drain(page, 0, page_private(page));
>  		} while (--to_free && --batch_free && !list_empty(list));
>  	}
> -- 1.7.6.1 .
> 
> 
> 
> .
> 
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org.  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

-- 
Kind regards,
Minchan Kim
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/