linux-kernel - Re: [linux-pm] [PATCH] hibernation should work ok with memory hotplug

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Date:	Wed, 05 Nov 2008 16:28:01 -0800
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Yasunori Goto <y-goto@...fujitsu.com>,
	Nigel Cunningham <ncunningham@...a.org.au>,
	Matt Tolentino <matthew.e.tolentino@...el.com>,
	linux-pm@...ts.osdl.org, Dave Hansen <haveblue@...ibm.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, pavel@...e.cz,
	Mel Gorman <mel@...net.ie>, Andy Whitcroft <apw@...dowen.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [linux-pm] [PATCH] hibernation should work ok with memory
	hotplug

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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