Re: Trying to make use of hotplug memory for xen balloon driver

[Jeremy Fitzhardinge <jeremy@goop.org>]

Dave Hansen wrote:
> The flags being all null looks highly suspicious to me.
>
> Once you've done an add_memory(), the new sections should show up
> in /sys.  Do you see them in there?
>
> Once they show up, you can online them with:
>
> 	echo online > /sys/devices/system/memory/memoryXXX/state
>
> That's what actually goes and mucks with the 'struct zone's and the
> pgdats to expand them.  It will also call online_page() on the whole
> range.  I think you're trying to do this manually, and missing part of
> it.  

Hm, actually this is precisely the wrong thing to do in this case.  When 
the balloon driver adds a new section of hotplug memory, its doing it to 
get the page structures, but there's no actual memory backing those 
pages.  The memory only comes into existence on a page-by-page basis 
when the balloon driver gets memory from the hypervisor and attaches it 
to each page (the balloon driver uses online_page() on each page as its 
ready).

If the user does a mass online via /sys the system explodes because it 
onlines a large number of pages which have no backing memory.  Since 
none of those pages can be mapped, the kernel explodes in a variety of 
interesting ways.

So I'd really like to inhibit the sysfs interface on these sections.  
Thoughts?

Thanks,
    J