Re: [PATCH] memory-hotplug: fix a drain pcp bug when offline pages

[Minchan Kim <minchan@kernel.org>]

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 .
> 
> 
> 
> .
> 
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-- 
Kind regards,
Minchan Kim

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