lkml.org 
[lkml]   [2008]   [Mar]   [29]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH RFC] hotplug-memory: refactor online_pages to separate zone growth from page onlining
Dave Hansen wrote:
> On Fri, 2008-03-28 at 19:08 -0700, Jeremy Fitzhardinge wrote:
>
>> My big remaining problem is how to disable the sysfs interface for this
>> memory. I need to prevent any onlining via /sys/device/system/memory.
>>
>
> I've been thinking about this some more, and I wish that you wouldn't
> just throw this interface away or completely disable it.

I had no intention of globally disabling it. I just need to disable it
for my use case.

> It actually
> does *exactly* what you want in a way. :)
>
> When the /memoryXX/ directory appears, that means that the hardware has
> found the memory, and that the 'struct page' is allocated and ready to
> be initialized.
>
> When the OS actually wants to use the memory (initialize the 'struct
> page', and free_page() it), it does the 'echo online > /sys...'. Both
> the 'struct page' and the memory represented by it are untouched until
> the "online". This was originally in place to avoid fragmenting it
> immediately in the case that the system did not need it.
>
> To me, it sounds like the only different thing that you want is to make
> sure that only partial sections are onlined. So, shall we work with the
> existing interfaces to online partial sections, or will we just disable
> it entirely when we see Xen?
>

Well, yes and no.

For the current balloon driver, it doesn't make much sense. It would
add a fair amount of complexity without any real gain. It's currently
based around alloc_page/free_page. When it wants to shrink the domain
and give memory back to the host, it allocates pages, adds the page
structures to a ballooned pages list, and strips off the backing memory
and gives it to the host. Growing the domain is the converse: it gets
pages from the host, pulls page structures off the list, binds them
together and frees them back to the kernel. If it runs out of ballooned
page structures, it hotplugs in some memory to add more.

That said, if (partial-)sections were much smaller - say 2-4 meg - and
page migration/defrag worked reliably, then we could probably do without
the balloon driver and do it all in terms of memory hot plug/unplug.
That would give us a general mechanism which could either be driven from
userspace, and/or have in-kernel Xen/kvm/s390/etc policy modules. Aside
from small sections, the only additional requirement would be an online
hook which can actually attach backing memory to the pages being
onlined, rather than just assuming an underlying DIMM as current code does.

> For Xen and KVM, how does it get decided that the guest needs more
> memory? Is this guest or host driven? Both? How is the guest
> notified? Is guest userspace involved at all?

In Xen, either the host or the guest can set the target size for the
domain, which is capped by the host-set limit. Aside from possibly
setting the target size, there's no usermode involvement in managing
ballooning. The virtio balloon driver is similar, though from a quick
look it seems to be entirely driven by the host side.

J


\
 
 \ /
  Last update: 2008-03-30 00:57    [W:0.076 / U:2.916 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site