xen-gntdev: Use ballooned pages for grant mappings

Grant mappings cause the PFN<->MFN mapping to be lost on the pages used
for the mapping. Instead of leaking memory, use pages that have already
been ballooned out and so have no valid mapping. This removes the need
for the bad-page leak workaround as pages are repopulated by the balloon
driver.

Acked-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/gntdev.c

@@ -36,6 +36,7 @@
 
 #include <xen/xen.h>
 #include <xen/grant_table.h>
+#include <xen/balloon.h>
 #include <xen/gntdev.h>
 #include <xen/events.h>
 #include <asm/xen/hypervisor.h>
@@ -122,10 +123,10 @@ static struct grant_map *gntdev_alloc_map(struct gntdev_priv *priv, int count)
 	    NULL == add->pages)
 		goto err;
 
+	if (alloc_xenballooned_pages(count, add->pages))
+		goto err;
+
 	for (i = 0; i < count; i++) {
-		add->pages[i] = alloc_page(GFP_KERNEL | __GFP_HIGHMEM);
-		if (add->pages[i] == NULL)
-			goto err;
 		add->map_ops[i].handle = -1;
 		add->unmap_ops[i].handle = -1;
 	}
@@ -137,11 +138,6 @@ static struct grant_map *gntdev_alloc_map(struct gntdev_priv *priv, int count)
 	return add;
 
 err:
-	if (add->pages)
-		for (i = 0; i < count; i++) {
-			if (add->pages[i])
-				__free_page(add->pages[i]);
-		}
 	kfree(add->pages);
 	kfree(add->grants);
 	kfree(add->map_ops);
@@ -184,8 +180,6 @@ static struct grant_map *gntdev_find_map_index(struct gntdev_priv *priv,
 
 static void gntdev_put_map(struct grant_map *map)
 {
-	int i;
-
 	if (!map)
 		return;
 
@@ -202,29 +196,7 @@ static void gntdev_put_map(struct grant_map *map)
 		if (!use_ptemod)
 			unmap_grant_pages(map, 0, map->count);
 
-		for (i = 0; i < map->count; i++) {
-			uint32_t check, *tmp;
-			if (!map->pages[i])
-				continue;
-			/* XXX When unmapping in an HVM domain, Xen will
-			 * sometimes end up mapping the GFN to an invalid MFN.
-			 * In this case, writes will be discarded and reads will
-			 * return all 0xFF bytes.  Leak these unusable GFNs
-			 * until Xen supports fixing their p2m mapping.
-			 *
-			 * Confirmed present in Xen 4.1-RC3 with HVM source
-			 */
-			tmp = kmap(map->pages[i]);
-			*tmp = 0xdeaddead;
-			mb();
-			check = *tmp;
-			kunmap(map->pages[i]);
-			if (check == 0xdeaddead)
-				__free_page(map->pages[i]);
-			else
-				pr_debug("Discard page %d=%ld\n", i,
-					page_to_pfn(map->pages[i]));
-		}
+		free_xenballooned_pages(map->count, map->pages);
 	}
 	kfree(map->pages);
 	kfree(map->grants);