linux-kernel - Re: [PATCH 2/2] Make is_mem_section_removable more conformable with offlining code

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Date:	Fri, 3 Sep 2010 10:25:58 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Hiroyuki Kamezawa <kamezawa.hiroyuki@...il.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Kleen, Andi" <andi.kleen@...el.com>,
	Haicheng Li <haicheng.li@...ux.intel.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Mel Gorman <mel@...ux.vnet.ibm.com>
Subject: Re: [PATCH 2/2] Make is_mem_section_removable more conformable
 with offlining code

On Fri 03-09-10 12:14:52, KAMEZAWA Hiroyuki wrote:
[...]
> ---
>  include/linux/memory_hotplug.h |    1 
>  mm/memory_hotplug.c            |   15 --------
>  mm/page_alloc.c                |   76 ++++++++++++++++++++++++++++++-----------
>  3 files changed, 59 insertions(+), 33 deletions(-)
> 
> Index: mmotm-0827/mm/page_alloc.c
> ===================================================================
> --- mmotm-0827.orig/mm/page_alloc.c
> +++ mmotm-0827/mm/page_alloc.c
> @@ -5274,11 +5274,63 @@ void set_pageblock_flags_group(struct pa
>   * page allocater never alloc memory from ISOLATE block.
>   */
>  
> +static int __count_unmovable_pages(struct zone *zone, struct page *page)
> +{
> +	unsigned long pfn, iter, found;
> +	/*
> +	 * For avoiding noise data, lru_add_drain_all() should be called.
> + 	 * before this.
> + 	 */
> +	if (zone_idx(zone) == ZONE_MOVABLE)
> +		return 0;

Cannot ZONE_MOVABLE contain different MIGRATE_types?

> +
> +	pfn = page_to_pfn(page);
> +	for (found = 0, iter = 0; iter < pageblock_nr_pages; iter++) {
> +		unsigned long check = pfn + iter;
> +
> +		if (!pfn_valid_within(check)) {
> +			iter++;
> +			continue;
> +		}
> +		page = pfn_to_page(check);
> +		if (!page_count(page)) {
> +			if (PageBuddy(page))

Why do you check page_count as well? PageBuddy has alwyas count==0,
right?

> +				iter += (1 << page_order(page)) - 1;
> +			continue;
> +		}
> +		if (!PageLRU(page))
> +			found++;
> +		/*
> +		 * If the page is not RAM, page_count()should be 0.
> +		 * we don't need more check. This is an _used_ not-movable page.
> +		 *
> +		 * The problematic thing here is PG_reserved pages. But if
> +		 * a PG_reserved page is _used_ (at boot), page_count > 1.
> +		 * But...is there PG_reserved && page_count(page)==0 page ?

Can we have PG_reserved && PG_lru? I also quite don't understand the
comment. At this place we are sure that the page is valid and neither
free nor LRU.

> +		 */
> +	}
> +	return found;
> +}
> +
> +bool is_pageblock_removable(struct page *page)
> +{
> +	struct zone *zone = page_zone(page);
> +	unsigned long flags;
> +	int num;
> +
> +	spin_lock_irqsave(&zone->lock, flags);
> +	num = __count_unmovable_pages(zone, page);
> +	spin_unlock_irqrestore(&zone->lock, flags);

Isn't this a problem? The function is triggered from userspace by sysfs
(0444 file) and holds the lock for pageblock_nr_pages. So someone can
simply read the file and block the zone->lock preventing/delaying
allocations for the rest of the system.

I think that the function should rather bail out as soon as possible.

[...]

>  	/* All pageblocks in the memory block are likely to be hot-removable */
> Index: mmotm-0827/include/linux/memory_hotplug.h
> ===================================================================
> --- mmotm-0827.orig/include/linux/memory_hotplug.h
> +++ mmotm-0827/include/linux/memory_hotplug.h
> @@ -76,6 +76,7 @@ extern int __add_pages(int nid, struct z
>  extern int __remove_pages(struct zone *zone, unsigned long start_pfn,
>  	unsigned long nr_pages);
>  
> +extern bool is_pageblock_removable(struct page *page);
>  #ifdef CONFIG_NUMA
>  extern int memory_add_physaddr_to_nid(u64 start);
>  #else

Shouldn't this go rather under CONFIG_MEMORY_HOTREMOVE?

Thanks!
-- 
Michal Hocko
L3 team 
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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