linux-kernel - Re: [PATCH 2/2] Make is_mem_section_removable more conformable with offlining code v3

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Date:	Mon, 6 Sep 2010 11:16:33 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Hiroyuki Kamezawa <kamezawa.hiroyuki@...il.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 v3

On Sat 04-09-10 10:55:16, Wu Fengguang wrote:
> On Fri, Sep 03, 2010 at 07:42:13PM +0800, Michal Hocko wrote:
> 
> > +/*
> > + * A free or LRU pages block are removable
> > + * Do not use MIGRATE_MOVABLE because it can be insufficient and
> > + * other MIGRATE types are tricky.
> > + * Do not hold zone->lock as this is used from user space by the
> > + * sysfs interface.
> > + */
> > +bool is_page_removable(struct page *page)
> > +{
> > +	int page_block = 1 << pageblock_order;
> > +
> > +	/* All pages from the MOVABLE zone are movable */
> > +	if (zone_idx(page_zone(page)) == ZONE_MOVABLE)
> > +		return true;
> > +
> > +	while (page_block > 0) {
> > +		int order = 0;
> > +
> > +		if (pfn_valid_within(page_to_pfn(page))) {
> > +			if (!page_count(page) && PageBuddy(page)) {
> 
> PageBuddy() is true only for the head page and false for all tail
> pages. So when is_page_removable() is given a random 4k page
> (get_any_page() will exactly do that), the above test is not enough.

OK, I haven't noticed that set_migratetype_isolate (which calls
is_page_removable in my patch - bellow) is called from that context.
is_mem_section_removable goes by pageblocks so we are always checking
the head.

Anyway, I can see why you are counting unmovable pages in your patch
now. You need it for notifier logic, so my approach is not usable.

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