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

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Date:	Wed, 15 Dec 2010 09:21:34 +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
Subject: Re: PROBLEM: __offline_isolated_pages may offline too many pages

On Tue, 14 Dec 2010 16:01:39 +0100
Ingo Korb <ingo@...na.de> wrote:

> Hi!
> 
> [1.] One line summary of the problem:
> __offline_isolated_pages may isolate too many pages
> 
> [2.] Full description of the problem/report:
> While experimenting with remove_memory/online_pages, removing as few 
> pages as possible (pageblock_nr_pages, 512 on my box) I noticed that the 
> number of pages marked "reserved" increased even though both functions 
> did not indicate an error. Following the code it was clear that 
> __offline_isolated_pages marked twice as many pages as it should:
> 

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.

> === start paste (from dmesg) ===
> Offlined Pages 512
> remove from free list c00 1024 e00
> === end paste ===
> 
> The issue seems to be that __offline_isolated_pages blindly uses 
> page_order() to determine how many pages it should mark as reserved in 
> the current loop iteration, without checking if this would exceed the 
> limit set by end_pfn.
> 

It's because designed to work under memory section, it's aligned to MAX_ORDER.
Its blindness works correctly.


> I'm not sure what the correct way to fix this would be - is memory 
> isolation supposed to touch the order of a page if it crosses the end 
> (or beginning!) of the range of pages to be isolated?
> 

Nothing to be fixed. If you _need_ another functionality, please add a new
feature. But, in theory, memory offline doesn't work in the range smaller
than MAX_ORDER because of buddy allocator.

Thanks,
-Kame

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