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 > > _PAGE_PRESENT. > > > > 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? > > http://lists.xenproject.org/archives/html/xen-devel/2014-04/msg01905.html > 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 SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>