drm/i915/gemfs: enable THP

Enable transparent-huge-pages through gemfs by mounting with
huge=within_size.

v2: sprinkle within_size comment
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171006145041.21673-4-matthew.auld@intel.comSigned-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171006221833.32439-3-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gemfs.c

@@ -24,6 +24,7 @@
 
 #include <linux/fs.h>
 #include <linux/mount.h>
+#include <linux/pagemap.h>
 
 #include "i915_drv.h"
 #include "i915_gemfs.h"
@@ -41,6 +42,27 @@ int i915_gemfs_init(struct drm_i915_private *i915)
 	if (IS_ERR(gemfs))
 		return PTR_ERR(gemfs);
 
+	/*
+	 * Enable huge-pages for objects that are at least HPAGE_PMD_SIZE, most
+	 * likely 2M. Note that within_size may overallocate huge-pages, if say
+	 * we allocate an object of size 2M + 4K, we may get 2M + 2M, but under
+	 * memory pressure shmem should split any huge-pages which can be
+	 * shrunk.
+	 */
+
+	if (has_transparent_hugepage()) {
+		struct super_block *sb = gemfs->mnt_sb;
+		char options[] = "huge=within_size";
+		int flags = 0;
+		int err;
+
+		err = sb->s_op->remount_fs(sb, &flags, options);
+		if (err) {
+			kern_unmount(gemfs);
+			return err;
+		}
+	}
+
 	i915->mm.gemfs = gemfs;
 
 	return 0;