Re: Handling NUMA page migration

[Frank Mehnert]

On Wednesday 05 June 2013 11:56:30 Michal Hocko wrote:
> On Wed 05-06-13 11:32:15, Frank Mehnert wrote:
> [...]
> 
> > Thank you very much for your help. As I said, this problem happens _only_
> > with NUMA_BALANCING enabled. I understand that you treat the VirtualBox
> > code as untrusted but the reason for the problem is that some assumption
> > is obviously not met: The VirtualBox code assumes that the memory it
> > allocates using case A and case B is
> > 
> >  1. always present and
> >  2. will always be backed by the same phyiscal memory
> > 
> > over the entire life time. Enabling NUMA_BALANCING seems to make this
> > assumption false. I only want to know why.
> 
> As I said earlier. Both the manual node migration and numa_fault handler
> do not migrate pages with elevated ref count (your A case) and pages
> that are not on the LRU. So if your Referenced pages might be on the LRU
> then you probably have to look into numamigrate_isolate_page and do an
> exception for PageReserved pages. But I am a bit suspicious this is the
> cause because the reclaim doesn't consider PageReserved pages either so
> they could get reclaimed. Or maybe you have handled that path in your
> kernel.

Thanks, I will also investigate into this direction.

> Or the other option is that you depend on a timing or something like
> that which doesn't hold anymore. That would be hard to debug though.
> 
> > I see, you don't believe me. I will add more code to the kernel logging
> > which pages were migrated.
> 
> Simple test for PageReserved flag in numamigrate_isolate_page should
> tell you more.
> 
> This would cover the migration part. Another potential problem could be
> that the page might get unmapped and marked for the numa fault (see
> do_numa_page). So maybe your code just assumes that the page even
> doesn't get unmapped?

Exactly, that's the assumption -- therefore all these vm_flags tricks.
If this assumption is wrong or not always true, can this requirement
(page is _never_ unmapped) be met at all?

Thanks,

Frank
-- 
Dr.-Ing. Frank Mehnert | Software Development Director, VirtualBox
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