On Tue 07-03-17 13:40:04, Igor Mammedov wrote:

On Mon, 6 Mar 2017 15:54:17 +0100
Michal Hocko [off-list ref] wrote:

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On Fri 03-03-17 18:34:22, Igor Mammedov wrote:

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in current mainline kernel it triggers following code path:

online_pages()
  ...
       if (online_type == MMOP_ONLINE_KERNEL) {                                 
                if (!zone_can_shift(pfn, nr_pages, ZONE_NORMAL, &zone_shift))    
                        return -EINVAL;  

Are you sure? I would expect MMOP_ONLINE_MOVABLE here

pretty much, reproducer is above so try and see for yourself

I will play with this...
 
[...]

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get_maintainer.pl doesn't lists linux-api for 31bc3858ea3e,
MAINTAINERS should be fixed if linux-api were to be CCed.  

user visible APIs _should_ be discussed at this mailing list regardless
what get_maintainer.pl says. This is not about who is the maintainer but
about getting as wide audience for things that would have to be
maintained basically for ever.

How would random contributor know which list to CC?

This should have been brought up during the review process which was
less than sufficient in this case.

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So unless this causes a major regression which would be hard to fix I
will submit the patch for inclusion.  

it will be a major regression due to lack of daemon that
could online fast and can't be killed on OOM. So this
clean up patch does break used feature without providing
a viable alternative.  

So let's discuss the current memory hotplug shortcomings and get rid of
the crud which developed on top. I will start by splitting up the patch
into 3 parts. Do the auto online thing from the HyperV and xen balloning
drivers and dropping the config option and finally drop the sysfs knob.
The last patch might be NAKed and I can live with that as long as the
reasoning is proper and there is a general consensus on that.

PS: CC me on that patches too

It's major regression if you remove auto online in kernels that
run on top of x86 kvm/vmware hypervisors, making API cleanups
while breaking useful functionality doesn't make sense.

I would ACK config option removal if auto online keeps working
for all x86 hypervisors (hyperv/xen isn't the only who needs it)
and keep kernel CLI option to override default.

That doesn't mean that others will agree with flipping default,
that's why config option has been added.

Now to sum up what's been discussed on this thread, there were 2
different issues discussed:
  1) memory hotplug: remove in kernel auto online for all
                     except of hyperv/xen

       - suggested RFC is not acceptable from virt point of view
         as it regresses guests on top of x86 kvm/vmware which
         both use ACPI based memory hotplug.

       - udev/userspace solution doesn't work in practice as it's
         too slow and unreliable when system is under load which
         is quite common in virt usecase. That's why auto online
         has been introduced in the first place.

Please try to be more specific why "too slow" is a problem. Also how
much slower are we talking about?
 

  2) memory unplug: online memory as movable

       - doesn't work currently with udev rule due to kernel
         issues https://bugzilla.redhat.com/show_bug.cgi?id=1314306#c7

These should be fixed
 

       - could be fixed both for in kernel auto online and udev
         with following patch:
         https://bugzilla.redhat.com/attachment.cgi?id=1146332
         but fixing it this way exposes zone disbalance issues,
         which are not present in current kernel as blocks are
         onlined in Zone Normal. So this is area to work and
         improve on.

       - currently if one wants to use online_movable,
         one has to either
           * disable auto online in kernel OR

which might not just work because an unmovable allocation could have
made the memblock pinned.

           * remove udev rule that distro ships
         AND write custom daemon that will be able to online
         block in right zone/order. So currently whole
         online_movable thing isn't working by default
         regardless of who onlines memory.

my epxperience with onlining full nodes as movable shows this works just
fine (with all the limitations of the movable zones but that is a
separate thing). I haven't played with configurations where movable
zones are sharing the node with other zones.

         I'm in favor of implementing that in kernel as it keeps
         kernel internals inside kernel and doesn't need
         kernel API to be involved (memory blocks in sysfs,
         online_kernel, online_movable)
         There would be no need in userspace which would have to
         deal with kernel zoo and maintain that as well.

The kernel is supposed to provide a proper API and that is sysfs
currently. I am not entirely happy about it either but pulling a lot of
code into the kernel is not the rigth thing to do. Especially when
different usecases require different treatment.
-- 
Michal Hocko
SUSE Labs

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