Subject: Re: [linux-pm] [PATCH] hibernation should work ok with memory
	hotplug
From: Dave Hansen <dave@linux.vnet.ibm.com>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>, Yasunori Goto <y-goto@jp.fujitsu.com>,
       Nigel Cunningham <ncunningham@crca.org.au>,
       Matt Tolentino <matthew.e.tolentino@intel.com>, linux-pm@lists.osdl.org,
       Dave Hansen <haveblue@us.ibm.com>, linux-kernel@vger.kernel.org,
       linux-mm@kvack.org, pavel@suse.cz, Mel Gorman <mel@skynet.ie>,
       Andy Whitcroft <apw@shadowen.org>,
       Andrew Morton <akpm@linux-foundation.org>
In-Reply-To: <20081106091441.6517c072.kamezawa.hiroyu@jp.fujitsu.com>
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	 <1225817945.12673.602.camel@nimitz>
	 <20081105093837.e073c373.kamezawa.hiroyu@jp.fujitsu.com>
	 <200811051208.26628.rjw@sisk.pl>
	 <20081106091441.6517c072.kamezawa.hiroyu@jp.fujitsu.com>
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Date: Wed, 05 Nov 2008 16:28:01 -0800
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On Thu, 2008-11-06 at 09:14 +0900, KAMEZAWA Hiroyuki wrote:
> Ok, please consider "when memory hotplug happens." 
> 
> In general, it happens when
>   1. memory is inserted to slot.
>   2. the firmware notifes the system to enable already inserted memory.
> 
> To trigger "1", you have to open cover of server/pc. Do you open pc while the system
> starts hibernation ? for usual people, no.

You're right, this won't happen very often.  We're trying to close a
theoretical hole that hasn't ever been observed in practice.  But, we
don't exactly leave races in code just because we haven't observed them.
I think this is a classic race.

If we don't close it now, then someone doing some really weirdo hotplug
is going to run into it at some point.  Who knows what tomorrow's
hardware/firmware will do?

> To trigger "2", the user have special console to tell firmware "enable this memory".
> Such firmware console or users have to know "the system works well." And, more important,
> when the system is suspended, the firmware can't do hotplug because the kernel is sleeping.
> So, such firmware console or operator have to know the system status.
> 
> Am I missing some ? Current linux can know PCI/USB hotplug while the
> system is suspended ?

* echo 'disk' > /sys/power/state
* count number of pages to write to disk
* turn all interrupts off
* copy pages to disk
* power down

I think the race we're trying to close is the one between when we count
pages and when we turn interrupts off.  I assume that there is a reason
that we don't do the *entire* hibernation process with interrupts off,
probably because it would "lock" the system up for too long, and can
even possibly fail.

-- Dave

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