Re: [Part3 PATCH v2 0/4] Support hot-remove local pagetable pages.

[Toshi Kani <toshi.kani@hp.com>]

On Wed, 2013-06-19 at 15:29 +0800, Tang Chen wrote:
> Hi Vasilis, Yinghai,
> 
> On 06/19/2013 01:05 AM, Vasilis Liaskovitis wrote:
> ......
> >
> > This could be a design problem of part3: if we allow local pagetable memory
> > to not be offlined but allow the offlining to return successfully, then
> > hot-remove is going to succeed. But the direct mapped pagetable pages are still
> > mapped in the kernel. The hot-removed memblocks will suddenly disappear (think
> > physical DIMMs getting disabled in real hardware, or in a VM case the
> > corresponding guest memory getting freed from the emulator e.g. qemu/kvm). The
> > system can crash as a result.
> >
> 
> Yes. Since the pagetable pages is only allocated to local node, a node may
> have more than one device, hot-remove only one memory device could be
> problematic.
> 
> But I think it will work if we hot-remove a whole node. I should have
> mentioned it. And sorry for the not fully test.
> 
> I think allocating pagetable pages to local device will resolve this problem.
> And need to restructure this patch-set.
> 
> > I think these local pagetables do need to be unmapped from kernel, offlined and
> > removed somehow - otherwise hot-remove should fail. Could they be migrated
> > alternatively e.g. to node 0 memory?  But Iiuc direct mapped pages cannot be
> > migrated, correct?
> 
> I think we have unmapped the local pagetables. in functions
> free_pud/pmd/pte_table(), we cleared pud, pmd, and pte. We just didn't
> free the pagetable pages to buddy.
> 
> But when we are not hot-removing the whole node, it is still problematic.
> This is true, and it is my design problem.
> 
> >
> > What is the original reason for local node pagetable allocation with regards
> > to memory hotplug? I assume we want to have hotplugged nodes use only their local
> > memory, so that there are no inter-node memory dependencies for hot-add/remove.
> > Are there other reasons that I am missing?
> 
> I think the original reason to do local node pagetable is to improve 
> performance.
> Using local pagetable, vmemmap and so on will be faster.
> 
> But actually I think there is no particular reason to implement memory hot-remove
> and local node pagetable at the same time. And before this patch-set, I also
> suggested once that implement memory hot-remove first, and then improve it to
> local pagetable. But Yinghai has done the local pagetable work in has 
> patches (part1).
> And my work is based on his patches. So I just did it.
> 
> But obviously it is more complicated than I thought.
> 
> And now, it seems tj has some more thinking on part1.
> 
> So how about the following plan:
> 1. Implement arranging hotpluggable memory with SRAT first, without 
> local pagetable.
>     (The main work in part2. And of course, need some patches in part1.)
> 2. Do the local device pagetable work, not local node.
> 3. Improve memory hotplug to support local device pagetable.

Sounds like a good plan.  Thanks Tang!
-Toshi


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