drm/i915/selftests: Limit live_gtt allocation test to fit within RAM

Limit the GTT size we try and allocate to ensure that it fits within RAM
and does not trigger the oomkiller indiscriminately.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180706125338.24432-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/selftests/i915_gem_gtt.c

@@ -148,7 +148,7 @@ static int igt_ppgtt_alloc(void *arg)
 {
 	struct drm_i915_private *dev_priv = arg;
 	struct i915_hw_ppgtt *ppgtt;
-	u64 size, last;
+	u64 size, last, limit;
 	int err = 0;
 
 	/* Allocate a ppggt and try to fill the entire range */
@@ -163,10 +163,18 @@ static int igt_ppgtt_alloc(void *arg)
 	if (!ppgtt->vm.allocate_va_range)
 		goto err_ppgtt_cleanup;
 
+	/*
+	 * While we only allocate the page tables here and so we could
+	 * address a much larger GTT than we could actually fit into
+	 * RAM, a practical limit is the amount of physical pages in the system.
+	 * This should ensure that we do not run into the oomkiller during
+	 * the test and take down the machine wilfully.
+	 */
+	limit = totalram_pages << PAGE_SHIFT;
+	limit = min(ppgtt->vm.total, limit);
+
 	/* Check we can allocate the entire range */
-	for (size = 4096;
-	     size <= ppgtt->vm.total;
-	     size <<= 2) {
+	for (size = 4096; size <= limit; size <<= 2) {
 		err = ppgtt->vm.allocate_va_range(&ppgtt->vm, 0, size);
 		if (err) {
 			if (err == -ENOMEM) {
@@ -183,9 +191,7 @@ static int igt_ppgtt_alloc(void *arg)
 	}
 
 	/* Check we can incrementally allocate the entire range */
-	for (last = 0, size = 4096;
-	     size <= ppgtt->vm.total;
-	     last = size, size <<= 2) {
+	for (last = 0, size = 4096; size <= limit; last = size, size <<= 2) {
 		err = ppgtt->vm.allocate_va_range(&ppgtt->vm,
 						  last, size - last);
 		if (err) {