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

From: Linus Torvalds
Date: Thu Feb 19 2015 - 18:09:10 EST

On Thu, Feb 19, 2015 at 5:06 AM, David Vrabel <david.vrabel@xxxxxxxxxx> wrote:
> 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.

That's some crazy stuff, but whatever. The patch is clearly good. Applied,

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