Re: Handling NUMA page migration

[Jerome Glisse]

 On Tue, Jun 04, 2013 at 02:14:45PM +0200, Frank Mehnert wrote:
> On Tuesday 04 June 2013 13:58:07 Robin Holt wrote:
> > This is probably more appropriate to be directed at the linux-mm
> > mailing list.
> > 
> > On Tue, Jun 04, 2013 at 09:22:10AM +0200, Frank Mehnert wrote:
> > > Hi,
> > > 
> > > our memory management on Linux hosts conflicts with NUMA page migration.
> > > I assume this problem existed for a longer time but Linux 3.8 introduced
> > > automatic NUMA page balancing which makes the problem visible on
> > > multi-node hosts leading to kernel oopses.
> > > 
> > > NUMA page migration means that the physical address of a page changes.
> > > This is fatal if the application assumes that this never happens for
> > > that page as it was supposed to be pinned.
> > > 
> > > We have two kind of pinned memory:
> > > 
> > > A) 1. allocate memory in userland with mmap()
> > > 
> > >    2. madvise(MADV_DONTFORK)
> > >    3. pin with get_user_pages().
> > >    4. flush dcache_page()
> > >    5. vm_flags |= (VM_DONTCOPY | VM_LOCKED)
> > >    
> > >       (resulting flags are VM_MIXEDMAP | VM_DONTDUMP | VM_DONTEXPAND |
> > >       
> > >        VM_DONTCOPY | VM_LOCKED | 0xff)
> > 
> > I don't think this type of allocation should be affected.  The
> > get_user_pages() call should elevate the pages reference count which
> > should prevent migration from completing.  I would, however, wait for
> > a more definitive answer.
> 
> Thanks Robin! Actually case B) is more important for us so I'm waiting
> for more feedback :)
> 
> Frank
> 
> > > B) 1. allocate memory with alloc_pages()
> > > 
> > >    2. SetPageReserved()
> > >    3. vm_mmap() to allocate a userspace mapping
> > >    4. vm_insert_page()
> > >    5. vm_flags |= (VM_DONTEXPAND | VM_DONTDUMP)
> > >    
> > >       (resulting flags are VM_MIXEDMAP | VM_DONTDUMP | VM_DONTEXPAND |
> > >       0xff)
> > > 
> > > At least the memory allocated like B) is affected by automatic NUMA page
> > > migration. I'm not sure about A).
> > > 
> > > 1. How can I prevent automatic NUMA page migration on this memory?
> > > 2. Can NUMA page migration also be handled on such kind of memory without
> > > 
> > >    preventing migration?
> > > 
> > > Thanks,
> > > 
> > > Frank

I was looking at migration code lately, and while i am not an expert at all
in this area. I think there is a bug in the way handle_mm_fault deals, or
rather not deals, with migration entry.

When huge page is migrated its pmd is replace with a special swp entry pmd,
which is a non zero pmd but that does not have any of the huge pmd flag set
so none of the handle_mm_fault path detect it as swap entry. Then believe
its a valid pmd and try to allocate pte under it which should oops.

Attached patch is what i believe should be done (not even compile tested).

Again i might be missing a subtelty somewhere else and just missed where
huge migration entry are dealt with.

Cheers,
Jerome