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Date:	Mon, 3 Nov 2008 23:39:17 +0000
From:	Andy Whitcroft <apw@...dowen.org>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Andrew Morton <akpm@...ux-foundation.org>, 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>
Subject: Re: [PATCH] hibernation should work ok with memory hotplug

On Mon, Nov 03, 2008 at 02:34:25PM -0800, Dave Hansen wrote:
> On Mon, 2008-11-03 at 23:24 +0100, Rafael J. Wysocki wrote:
> > On Monday, 3 of November 2008, Dave Hansen wrote:
> > > 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.
> > 
> > This operation is carried out on one CPU with interrupts disabled.  Is that
> > not enough?
> 
> If that's true then you don't need any locking for anything at all,
> right?
> 
> All of the changes I was talking about occur inside the kernel and code
> has to run for it to happen.  So, if you are saying that absolutely no
> other code on the system can possibly run, then it should be OK.
> 
> > > 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.
> > 
> > Can that really happen given the conditions above?
> 
> Nope.
> 
> But, as I think about it, there is another issue that we need to
> address, CONFIG_NODES_SPAN_OTHER_NODES.
> 
> A node might have a node_start_pfn=0 and a node_end_pfn=100 (and it may
> have only one zone).  But, there may be another node with
> node_start_pfn=10 and a node_end_pfn=20.  This loop:
> 
>         for_each_zone(zone) {
> 		...
>                 for (pfn = zone->zone_start_pfn; pfn < max_zone_pfn; pfn++)
>                         if (page_is_saveable(zone, pfn))
>                                 memory_bm_set_bit(orig_bm, pfn);
>         }
> 
> will walk over the smaller node's pfn range multiple times.  Is this OK?
> 
> I think all you have to do to fix it is check page_zone(page) == zone
> and skip out if they don't match.
> 
> Andy, does anything else stick out to you?

I agree that there needs to be a check for being in the zone there to
avoid the overlapping nodes issue.  Also need to make sure when
constructing that check we check for pfn_valid before looking at the
page to avoid holes in the memmap.

-apw
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