Re: [PATCH] hibernation should work ok with memory hotplug

[Dave Hansen <dave@linux.vnet.ibm.com>]

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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@sisk.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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From: Dave Hansen <haveblue@us.ibm.com>
To: swsusp@lister.fornax.hu
Cc: lhms <lhms-devel@lists.sourceforge.net>
Subject: memory hotplug and software suspend
Date: Mon, 13 Jun 2005 10:08:55 -0700
Message-ID: <1118682535.22631.22.camel@localhost>

Software suspend folks,

We're getting ready to submit memory hot-addition to the mainline
kernel.  The patches that we currently have work with everything, except
for the software suspend code currently in the kernel.

The issue is that, during a swsusp operation, a hardware memory hotplug
operation may occur.  For now, let's think about memory addition, and
ignore removal.

For example, let's look at copy_data_pages():

        for_each_zone(zone) {
		...
                for (zone_pfn = 0; zone_pfn < zone->spanned_pages; ++zone_pfn) {

In theory, while this loop is going on, a zone might be added, or a
zone's range might be expanded, making zone_pfn not start at the
beginning of a zone.  I think that can be fixed by using a new seqlock
that I plan to submit:

http://www.sr71.net/patches/2.6.12/2.6.12-rc5-mhp1/broken-out/C6-zone-span_seqlock.patch

and holding the new pgdat->size_lock around the call to the saveable()
function, to keep the pfn_valid().

http://www.sr71.net/patches/2.6.12/2.6.12-rc5-mhp1/broken-out/C5.2-pgdat_size_lock.patch

So, I have a couple of questions about swsusp.  Is the current code in
2.6.12-rc6 going to be around for a while?  If not, does anyone have a
problem with me making MEMORY_HOTPLUG depend on !SOFTWARE_SUSPEND for a
bit, until everything settles out, and we can fix them to work together
later?

-- Dave