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

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Date:	Wed, 5 Nov 2008 12:08:25 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Dave Hansen <dave@...ux.vnet.ibm.com>,
	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 Wednesday, 5 of November 2008, KAMEZAWA Hiroyuki wrote:
> On Tue, 04 Nov 2008 08:59:05 -0800
> Dave Hansen <dave@...ux.vnet.ibm.com> wrote:
> 
> > On Tue, 2008-11-04 at 17:34 +0100, Rafael J. Wysocki wrote:
> > > Now, I need to do one more thing, which is to check how much memory has to be
> > > freed before creating the image.  For this purpose I need to lock memory
> > > hotplug temporarily, count pages to free and unlock it.  What interface should
> > > I use for this purpose? 
> > > 
> > > [I'll also need to lock memory hotplug temporarily during resume.]
> > 
> > We currently don't have any big switch to disable memory hotplug, like
> > lock_memory_hotplug() or something. :)
> > 
> > If you are simply scanning and counting pages, I think the best thing to
> > use would be the zone_span_seq*() seqlock stuff.  Do your count inside
> > the seqlock's while loop.  That covers detecting a zone changing while
> > it is being scanned.
> > 
> > The other case to detect is when a new zone gets added.  These are
> > really rare.  Rare enough that we actually use a stop_machine() call in
> > build_all_zonelists() to do it.  All you would have to do is detect when
> > one of these calls gets made.  I think that's a good application for a
> > new seq_lock.
> > 
> > I've attached an utterly untested patch that should do the trick.
> > Yasunori and KAME should probably take a look at it since the node
> > addition code is theirs.
> > 
> 
> Hmm ? I think there is no real requirement for doing hibernation while
> memory is under hotplug.
> 
> Assume following.
>  - memory hotplug can be triggerred by
>     1. interrupt from system.
>     2. "probe" interface in sysfs.
>  - ONLINE/OFFLINE is only trigerred by sysfs interface.
> 
> I believe we can't block "1", but "1" cannot be raised while hibernation.
> (If it happens, it's mistake of the firmware.)
> 
> "probe" interface can be triggered from userland. Then it may be worth to be
> blocked. How about to add device_pm_lock() to following place ?

This is not necessary as long as we freeze the userland before hibernation.
Still, this is one thing to remeber that the freezing is needed for. :-)

1. seems to be problematic, though, since we rely on zones remaining
unchanged while we're counting memory pages to free before hibernation
and this happens before the calling ->suspend() methods of device drivers.
Of course we can count free pages in a different way, but that will be a
substantial modification (I think).

How's the firmware supposed to be notified that hibernation is going to happen?

Rafael
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