linux-kernel - Re: PROBLEM: __offline_isolated_pages may offline too many pages

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Date:	Thu, 16 Dec 2010 09:06:57 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Ingo Korb <ingo@...na.de>
Cc:	linux-mm@...ck.org, akpm@...ux-foundation.org, mel@....ul.ie,
	cl@...ux-foundation.org, yinghai@...nel.org, andi.kleen@...el.com,
	linux-kernel@...r.kernel.org,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: PROBLEM: __offline_isolated_pages may offline too many pages

On Wed, 15 Dec 2010 10:25:51 +0100
Ingo Korb <ingo@...na.de> wrote:

> On 15.12.2010 01:21, KAMEZAWA Hiroyuki wrote:
> 
> > It's designed for offline memory section>  MAX_ORDER. pageblock_nr_pages
> > is tend to be smaller than that.
> >
> > Do you see the problem with _exsisting_ user interface of memory hotplug ?
> > I think we have no control other than memory section.
> 
> The existing, exported interface (remove_memory() - the check itself is 
> in offline_pages()) only checks if both start and end of the 
> to-be-removed block are aligned to pageblock_nr_pages. As you noted the 
> actual size and alignment requirements in __offline_isolated_pages can 
> be larger that that, so I think the checks in offline_pages() should be 
> changed (if 1<<MAX_ORDER is always >= pageblock_nr_pages) or extended 
> (if there can be any relation between the two).
> 

Ok, maybe my mistake. This is a fix. Thank you for reporting.
==

offline_pages()'s sanity check of given range is wrong. It should
be aligned to MAX_ORDER. Current exsiting caller uses SECTION_SIZE
alignment, so this change has no influence to exsisting callers.

Reported-by: Ingo Korb <ingo@...na.de>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
---
 mm/memory_hotplug.c |   10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

Index: linux-2.6.37-rc5/mm/memory_hotplug.c
===================================================================
--- linux-2.6.37-rc5.orig/mm/memory_hotplug.c
+++ linux-2.6.37-rc5/mm/memory_hotplug.c
@@ -798,10 +798,14 @@ static int offline_pages(unsigned long s
 	struct memory_notify arg;
 
 	BUG_ON(start_pfn >= end_pfn);
-	/* at least, alignment against pageblock is necessary */
-	if (!IS_ALIGNED(start_pfn, pageblock_nr_pages))
+	/*
+	 * Considering buddy allocator which joins nearby pages, the range
+	 * in offline should be aligned to MAX_ORDER. If not, isolated
+	 * page will be joined to other (not isolated) pages.
+	 */
+	if (!IS_ALIGNED(start_pfn, MAX_ORDER_NR_PAGES))
 		return -EINVAL;
-	if (!IS_ALIGNED(end_pfn, pageblock_nr_pages))
+	if (!IS_ALIGNED(end_pfn, MAX_ORDER_NR_PAGES))
 		return -EINVAL;
 	/* This makes hotplug much easier...and readable.
 	   we assume this for now. .*/

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