Re: NUMA_BALANCING and Xen PV guest regression in 3.20-rc0

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Thu, Feb 19, 2015 at 01:06:53PM +0000, David Vrabel wrote:
> Mel,
> 
> The NUMA_BALANCING series beginning with 5d833062139d (mm: numa: do not
> dereference pmd outside of the lock during NUMA hinting fault) and
> specifically 8a0516ed8b90 (mm: convert p[te|md]_numa users to
> p[te|md]_protnone_numa) breaks Xen 64-bit PV guests.
> 
> Any fault on a present userspace mapping (e.g., a write to a read-only
> mapping) is being misinterpreted as a NUMA hinting fault and not handled
> correctly.  All userspace programs end up continuously  faulting.
> 
> This is because the hypervisor sets _PAGE_GLOBAL (== _PAGE_PROTNONE) on
> all present userspace page table entries.

I'm feeling I miss very basic background on how Xen works, but why does it
set _PAGE_GLOBAL on userspace entries? It sounds strange to me.

> 
> Note that the comment in asm/pgtable_types.h that says that
> _PAGE_BIT_PROTNONE is only valid on non-present entries.
> 
>   /* If _PAGE_BIT_PRESENT is clear, we use these: */
>   /* - if the user mapped it with PROT_NONE; pte_present gives true */
>   #define _PAGE_BIT_PROTNONE	_PAGE_BIT_GLOBAL
> 
> Adjusting pte_protnone() and pmd_protnone() to check for the absence of
> _PAGE_PRESENT allows 64-bit Xen PV guests to work correctly again (see
> following patch), but I'm not sure if NUMA_BALANCING would correctly
> work with this change.
-- 
 Kirill A. Shutemov