The attached patch does a lock-break in copy_page_range() if needed, to
reduce scheduling latencies.

has been tested as part of the -VP patchset.

Signed-off-by: Ingo Molnar <mingo@elte.hu>

--- linux/mm/memory.c.orig	
+++ linux/mm/memory.c	
@@ -337,6 +337,25 @@ cont_copy_pte_range_noset:
 				}
 				src_pte++;
 				dst_pte++;
+				/*
+				 * We are holding two locks at this point -
+				 * any of them could generate latencies in
+				 * another task on another CPU:
+				 */
+				if (need_resched() ||
+					need_lockbreak(&src->page_table_lock) ||
+					need_lockbreak(&dst->page_table_lock))
+				{
+					pte_unmap_nested(src_pte);
+					pte_unmap(dst_pte);
+					spin_unlock(&src->page_table_lock);
+					spin_unlock(&dst->page_table_lock);
+					cond_resched();
+					spin_lock(&dst->page_table_lock);
+					spin_lock(&src->page_table_lock);
+					dst_pte = pte_offset_map(dst_pmd, address);
+					src_pte = pte_offset_map_nested(src_pmd, address);
+				}
 			} while ((unsigned long)src_pte & PTE_TABLE_MASK);
 			pte_unmap_nested(src_pte-1);
 			pte_unmap(dst_pte-1);