Subject: Add fallback when XENMEM_exchange fails to replace contiguous region
From: jbeulich@novell.com
Patch-mainline: obsolete
References: 181869

This avoids losing precious special memory in places where any memory can be
used.

--- head-2009-06-23.orig/arch/x86/mm/hypervisor.c	2009-06-23 13:47:52.000000000 +0200
+++ head-2009-06-23/arch/x86/mm/hypervisor.c	2009-06-09 15:52:17.000000000 +0200
@@ -43,6 +43,7 @@
 #include <xen/interface/memory.h>
 #include <linux/module.h>
 #include <linux/percpu.h>
+#include <linux/highmem.h>
 #include <asm/tlbflush.h>
 #include <linux/highmem.h>
 
@@ -719,6 +720,83 @@ void xen_destroy_contiguous_region(unsig
 		BUG();
 
 	balloon_unlock(flags);
+
+	if (unlikely(!success)) {
+		/* Try hard to get the special memory back to Xen. */
+		exchange.in.extent_order = 0;
+		set_xen_guest_handle(exchange.in.extent_start, &in_frame);
+
+		for (i = 0; i < (1U<<order); i++) {
+			struct page *page = alloc_page(__GFP_HIGHMEM|__GFP_COLD);
+			unsigned long pfn;
+			mmu_update_t mmu;
+			unsigned int j = 0;
+
+			if (!page) {
+				printk(KERN_WARNING "Xen and kernel out of memory "
+				       "while trying to release an order %u "
+				       "contiguous region\n", order);
+				break;
+			}
+			pfn = page_to_pfn(page);
+
+			balloon_lock(flags);
+
+			if (!PageHighMem(page)) {
+				void *v = __va(pfn << PAGE_SHIFT);
+
+				scrub_pages(v, 1);
+				MULTI_update_va_mapping(cr_mcl + j, (unsigned long)v,
+							__pte_ma(0), UVMF_INVLPG|UVMF_ALL);
+				++j;
+			}
+#ifdef CONFIG_XEN_SCRUB_PAGES
+			else {
+				scrub_pages(kmap(page), 1);
+				kunmap(page);
+				kmap_flush_unused();
+			}
+#endif
+
+			frame = pfn_to_mfn(pfn);
+			set_phys_to_machine(pfn, INVALID_P2M_ENTRY);
+
+			MULTI_update_va_mapping(cr_mcl + j, vstart,
+						pfn_pte_ma(frame, PAGE_KERNEL),
+						UVMF_INVLPG|UVMF_ALL);
+			++j;
+
+			pfn = __pa(vstart) >> PAGE_SHIFT;
+			set_phys_to_machine(pfn, frame);
+			if (!xen_feature(XENFEAT_auto_translated_physmap)) {
+				mmu.ptr = ((uint64_t)frame << PAGE_SHIFT) | MMU_MACHPHYS_UPDATE;
+				mmu.val = pfn;
+				cr_mcl[j].op = __HYPERVISOR_mmu_update;
+				cr_mcl[j].args[0] = (unsigned long)&mmu;
+				cr_mcl[j].args[1] = 1;
+				cr_mcl[j].args[2] = 0;
+				cr_mcl[j].args[3] = DOMID_SELF;
+				++j;
+			}
+
+			cr_mcl[j].op = __HYPERVISOR_memory_op;
+			cr_mcl[j].args[0] = XENMEM_decrease_reservation;
+			cr_mcl[j].args[1] = (unsigned long)&exchange.in;
+
+			if (HYPERVISOR_multicall(cr_mcl, j + 1))
+				BUG();
+			BUG_ON(cr_mcl[j].result != 1);
+			while (j--)
+				BUG_ON(cr_mcl[j].result != 0);
+
+			balloon_unlock(flags);
+
+			free_empty_pages(&page, 1);
+
+			in_frame++;
+			vstart += PAGE_SIZE;
+		}
+	}
 }
 EXPORT_SYMBOL_GPL(xen_destroy_contiguous_region);
 
--- head-2009-06-23.orig/drivers/xen/balloon/balloon.c	2009-06-26 14:48:15.000000000 +0200
+++ head-2009-06-23/drivers/xen/balloon/balloon.c	2009-06-26 14:48:31.000000000 +0200
@@ -698,7 +698,8 @@ struct page **alloc_empty_pages_and_page
 	goto out;
 }
 
-void free_empty_pages_and_pagevec(struct page **pagevec, int nr_pages)
+static void _free_empty_pages_and_pagevec(struct page **pagevec, int nr_pages,
+					  int free_vec)
 {
 	unsigned long flags;
 	int i;
@@ -709,15 +710,28 @@ void free_empty_pages_and_pagevec(struct
 	balloon_lock(flags);
 	for (i = 0; i < nr_pages; i++) {
 		BUG_ON(page_count(pagevec[i]) != 1);
-		balloon_append(pagevec[i], 0);
+		balloon_append(pagevec[i], !free_vec);
 	}
+	if (!free_vec)
+		totalram_pages = bs.current_pages -= nr_pages;
 	balloon_unlock(flags);
 
-	kfree(pagevec);
+	if (free_vec)
+		kfree(pagevec);
 
 	schedule_work(&balloon_worker);
 }
 
+void free_empty_pages_and_pagevec(struct page **pagevec, int nr_pages)
+{
+	_free_empty_pages_and_pagevec(pagevec, nr_pages, 1);
+}
+
+void free_empty_pages(struct page **pagevec, int nr_pages)
+{
+	_free_empty_pages_and_pagevec(pagevec, nr_pages, 0);
+}
+
 void balloon_release_driver_page(struct page *page)
 {
 	unsigned long flags;
--- head-2009-06-23.orig/include/xen/balloon.h	2009-06-23 13:28:00.000000000 +0200
+++ head-2009-06-23/include/xen/balloon.h	2009-06-09 15:52:17.000000000 +0200
@@ -47,6 +47,10 @@ void balloon_update_driver_allowance(lon
 struct page **alloc_empty_pages_and_pagevec(int nr_pages);
 void free_empty_pages_and_pagevec(struct page **pagevec, int nr_pages);
 
+/* Free an empty page range (not allocated through
+   alloc_empty_pages_and_pagevec), adding to the balloon. */
+void free_empty_pages(struct page **pagevec, int nr_pages);
+
 void balloon_release_driver_page(struct page *page);
 
 /*