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    SubjectRe: [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@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.
    > >
    > > 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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