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.orig/arch/x86/mm/hypervisor.c	2012-05-31 14:45:36.000000000 +0200
+++ head/arch/x86/mm/hypervisor.c	2012-06-01 15:06:05.000000000 +0200
@@ -774,6 +774,77 @@ void xen_destroy_contiguous_region(unsig
 		BUG();
 
 	balloon_unlock(flags);
+
+	if (likely(success))
+		return;
+
+	/* 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) {
+			pr_warn("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);
+
+			xen_scrub_pages(v, 1);
+			MULTI_update_va_mapping(cr_mcl + j++, (unsigned long)v,
+						__pte_ma(0),
+						UVMF_INVLPG|UVMF_ALL);
+#ifdef CONFIG_XEN_SCRUB_PAGES
+		} else {
+			xen_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);
+
+		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;
+			MULTI_mmu_update(cr_mcl + j++, &mmu, 1, 0, DOMID_SELF);
+		}
+
+		MULTI_memory_op(cr_mcl + j, XENMEM_decrease_reservation,
+				&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.orig/drivers/xen/balloon/balloon.c	2012-06-08 11:27:49.000000000 +0200
+++ head/drivers/xen/balloon/balloon.c	2012-06-08 11:28:45.000000000 +0200
@@ -744,28 +744,45 @@ struct page **alloc_empty_pages_and_page
 }
 EXPORT_SYMBOL_GPL(alloc_empty_pages_and_pagevec);
 
-void free_empty_pages_and_pagevec(struct page **pagevec, int nr_pages)
+#endif /* CONFIG_XEN_BACKEND */
+
+#ifdef CONFIG_XEN
+static void _free_empty_pages(struct page **pagevec, int nr_pages,
+			      bool account)
 {
 	unsigned long flags;
 	int i;
 
-	if (pagevec == NULL)
-		return;
-
 	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], account);
+	}
+	if (account) {
+		bs.current_pages -= nr_pages;
+		totalram_pages = bs.current_pages - totalram_bias;
 	}
 	balloon_unlock(flags);
 
-	kfree(pagevec);
-
 	schedule_work(&balloon_worker);
 }
-EXPORT_SYMBOL_GPL(free_empty_pages_and_pagevec);
 
-#endif /* CONFIG_XEN_BACKEND */
+void free_empty_pages(struct page **pagevec, int nr_pages)
+{
+	_free_empty_pages(pagevec, nr_pages, true);
+}
+#endif
+
+#if IS_ENABLED(CONFIG_XEN_BACKEND)
+void free_empty_pages_and_pagevec(struct page **pagevec, int nr_pages)
+{
+	if (pagevec) {
+		_free_empty_pages(pagevec, nr_pages, false);
+		kfree(pagevec);
+	}
+}
+EXPORT_SYMBOL_GPL(free_empty_pages_and_pagevec);
+#endif
 
 void balloon_release_driver_page(struct page *page)
 {
--- head.orig/include/xen/balloon.h	2012-02-09 12:32:50.000000000 +0100
+++ head/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);
 
 /*