Re: Handling NUMA page migration

[Michal Hocko]

On Wed 05-06-13 03:22:32, Frank Mehnert wrote:
> 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?

yes, just pin the page by get_page(). Reserved pages are usually not
touched because they are not sitting in the LRU (that just doesn't make
any sense) - why we would age such pages in the first place.
-- 
Michal Hocko
SUSE Labs
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