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SubjectRe: [PATCH 4/5] mm: use paravirt friendly ops for NUMA hinting ptes
On Tue, Apr 15, 2014 at 11:27:56AM +0100, David Vrabel wrote:
> On 08/04/14 14:09, Mel Gorman wrote:
> > David Vrabel identified a regression when using automatic NUMA balancing
> > under Xen whereby page table entries were getting corrupted due to the
> > use of native PTE operations. Quoting him
> >
> > Xen PV guest page tables require that their entries use machine
> > addresses if the preset bit (_PAGE_PRESENT) is set, and (for
> > successful migration) non-present PTEs must use pseudo-physical
> > addresses. This is because on migration MFNs in present PTEs are
> > translated to PFNs (canonicalised) so they may be translated back
> > to the new MFN in the destination domain (uncanonicalised).
> >
> > pte_mknonnuma(), pmd_mknonnuma(), pte_mknuma() and pmd_mknuma()
> > set and clear the _PAGE_PRESENT bit using pte_set_flags(),
> > pte_clear_flags(), etc.
> >
> > In a Xen PV guest, these functions must translate MFNs to PFNs
> > when clearing _PAGE_PRESENT and translate PFNs to MFNs when setting
> >
> > His suggested fix converted p[te|md]_[set|clear]_flags to using
> > paravirt-friendly ops but this is overkill. He suggested an alternative of
> > using p[te|md]_modify in the NUMA page table operations but this is does
> > more work than necessary and would require looking up a VMA for protections.
> >
> > This patch modifies the NUMA page table operations to use paravirt friendly
> > operations to set/clear the flags of interest. Unfortunately this will take
> > a performance hit when updating the PTEs on CONFIG_PARAVIRT but I do not
> > see a way around it that does not break Xen.
> We're getting more reports of users hitting this regression with distro
> provided kernels. Irrespective of the rest of this series, can we get
> at least this applied and tagged for stable, please?

The resending of the series got delayed until today. Fengguang Wu hit
problems testing the series and I ran into a number of similarly shaped
problems that took time to resolve. I sent out a v4 of the series with this
patch at the front and a note on the leader saying it should be picked up
for stable regardless of what happens with the patches 2 and 3.

Mel Gorman

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