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Date:	Mon, 03 Nov 2008 13:21:48 -0800
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>, pavel@...e.cz,
	linux-kernel@...r.kernel.org, linux-pm@...ts.osdl.org,
	Matt Tolentino <matthew.e.tolentino@...el.com>,
	Dave Hansen <haveblue@...ibm.com>, linux-mm@...ck.org,
	Mel Gorman <mel@...net.ie>, Andy Whitcroft <apw@...dowen.org>
Subject: Re: [PATCH] hibernation should work ok with memory hotplug

On Mon, 2008-11-03 at 12:51 -0800, Andrew Morton wrote:
> On Wed, 29 Oct 2008 13:25:00 +0100
> "Rafael J. Wysocki" <rjw@...k.pl> wrote:
> > On Wednesday, 29 of October 2008, Pavel Machek wrote:
> > > 
> > > hibernation + memory hotplug was disabled in kconfig because we could
> > > not handle hibernation + sparse mem at some point. It seems to work
> > > now, so I guess we can enable it.
> > 
> > OK, if "it seems to work now" means that it has been tested and confirmed to
> > work, no objection from me.
> 
> yes, that was not a terribly confidence-inspiring commit message.
> 
> 3947be1969a9ce455ec30f60ef51efb10e4323d1 said "For now, disable memory
> hotplug when swsusp is enabled.  There's a lot of churn there right
> now.  We'll fix it up properly once it calms down." which is also
> rather rubbery.  
> 
> Cough up, guys: what was the issue with memory hotplug and swsusp, and
> is it indeed now fixed?

I suck.  That commit message was horrid and I'm racking my brain now to
remember what I meant.  Don't end up like me, kids.

I've attached the message that I sent to the swsusp folks.  I never got
a reply from that as far as I can tell.

http://sourceforge.net/mailarchive/forum.php?thread_name=1118682535.22631.22.camel%40localhost&forum_name=lhms-devel

As I look at it now, it hasn't improved much since 2005.  Take a look at
kernel/power/snapshot.c::copy_data_pages().  It still assumes that the
list of zones that a system has is static.  Memory hotplug needs to be
excluded while that operation is going on.

page_is_saveable() checks for pfn_valid().  But, with memory hotplug,
things can become invalid at any time since no references are held or
taken on the page.  Or, a page that *was* invalid may become valid and
get missed.

The "missing a page" thing is probably correctable via the
zone_span_seqbegin() locks.  The "page becoming invalid" thing is
probably mostly fixable by acquiring a reference to the page itself.
I'd need to look how the locking on the hot remove side is working these
days to be much more constructive than that.

-- Dave

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