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

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

On Wed, 2013-06-19 at 11:58 +0900, Yasuaki Ishimatsu wrote:
> 2013/06/19 8:59, Toshi Kani wrote:
> > 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?
> 
> If my understanding is correct:
> Main purpose of Yinghai's work is to put pagetable on local node ram.
> For this, he needs to know SRAT information before setting pagetable.
> So part1 does them same time.

Thanks Yasuaki.  I like Tang Chen's new plan, and I think it is going to
be easier to proceed in that way.
-Toshi


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