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

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

On Tue, 2013-06-18 at 19:05 +0200, Vasilis Liaskovitis wrote:
> Hi,
> 
> On Thu, Jun 13, 2013 at 09:03:52PM +0800, Tang Chen wrote:
> > The following patch-set from Yinghai allocates pagetables to local nodes.
> > v1: https://lkml.org/lkml/2013/3/7/642
> > v2: https://lkml.org/lkml/2013/3/10/47
> > v3: https://lkml.org/lkml/2013/4/4/639
> > v4: https://lkml.org/lkml/2013/4/11/829
> > 
> > Since pagetable pages are used by the kernel, they cannot be offlined.
> > As a result, they cannot be hot-remove.
> > 
> > This patch fix this problem with the following solution:
> > 
> >      1.   Introduce a new bootmem type LOCAL_NODE_DATAL, and register local
> >           pagetable pages as LOCAL_NODE_DATAL by setting page->lru.next to
> >           LOCAL_NODE_DATAL, just like we register SECTION_INFO pages.
> > 
> >      2.   Skip LOCAL_NODE_DATAL pages in offline/online procedures. When the
> >           whole memory block they reside in is offlined, the kernel can
> >           still access the pagetables.
> >           (This changes the semantics of offline/online a little bit.)
> 
> 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.
> 
> 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?
> 
> 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 second Vasilis.  The part1/2/3 series could be much simpler & less
riskier if we focus on the SRAT changes first, and make the local node
pagetable changes as a separate item.  Is there particular reason why
they have to be done at a same time?

Thanks,
-Toshi


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