[PATCH] do not set extents feature from the kernel

[Eric Sandeen <sandeen@redhat.com>]

We've talked for a while about getting rid of any feature-
setting from the kernel; this gets rid of the code which would
set the INCOMPAT_EXTENTS flag on the first file write when mounted
as ext4[dev].

With this patch, if the extents feature is not already set on disk,
then mounting as ext4 will fall back to noextents with a warning,
and if -o extents is explicitly requested, the mount will fail,
also with warning.

Does this seem like a decent approach?

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
---

Index: linux-2.6/fs/ext4/ialloc.c
===================================================================
--- linux-2.6.orig/fs/ext4/ialloc.c	2008-07-07 10:22:00.000000000 -0500
+++ linux-2.6/fs/ext4/ialloc.c	2008-07-07 12:07:02.133292077 -0500
@@ -740,14 +740,10 @@ got:
 		goto fail_free_drop;
 
 	if (test_opt(sb, EXTENTS)) {
-		/* set extent flag only for diretory, file and normal symlink*/
+		/* set extent flag only for directory, file and normal symlink*/
 		if (S_ISDIR(mode) || S_ISREG(mode) || S_ISLNK(mode)) {
 			EXT4_I(inode)->i_flags |= EXT4_EXTENTS_FL;
 			ext4_ext_tree_init(handle, inode);
-			err = ext4_update_incompat_feature(handle, sb,
-					EXT4_FEATURE_INCOMPAT_EXTENTS);
-			if (err)
-				goto fail_free_drop;
 		}
 	}
 
Index: linux-2.6/fs/ext4/super.c
===================================================================
--- linux-2.6.orig/fs/ext4/super.c	2008-07-07 10:22:19.000000000 -0500
+++ linux-2.6/fs/ext4/super.c	2008-07-07 14:08:43.684291817 -0500
@@ -1309,6 +1309,13 @@ set_qf_format:
 			clear_opt(sbi->s_mount_opt, NOBH);
 			break;
 		case Opt_extents:
+			if (!EXT4_HAS_INCOMPAT_FEATURE(sb,
+					EXT4_FEATURE_INCOMPAT_EXTENTS)) {
+				ext4_warning(sb, __func__,
+					"extents feature not enabled "
+					"on this filesystem, use tune2fs\n");
+				return 0;
+			}
 			set_opt (sbi->s_mount_opt, EXTENTS);
 			break;
 		case Opt_noextents:
@@ -1919,12 +1926,18 @@ static int ext4_fill_super (struct super
 
 	/*
 	 * turn on extents feature by default in ext4 filesystem
-	 * User -o noextents to turn it off
+	 * only if feature flag already set by mkfs or tune2fs.
+	 * Use -o noextents to turn it off
 	 */
-	set_opt(sbi->s_mount_opt, EXTENTS);
+	if (EXT4_HAS_INCOMPAT_FEATURE(sb, EXT4_FEATURE_INCOMPAT_EXTENTS))
+		set_opt(sbi->s_mount_opt, EXTENTS);
+	else
+		ext4_warning(sb, __func__,
+			"extents feature not enabled on this filesystem, "
+			"use tune2fs.\n");
 	/*
-	 * turn on mballoc feature by default in ext4 filesystem
-	 * User -o nomballoc to turn it off
+	 * turn on mballoc code by default in ext4 filesystem
+	 * Use -o nomballoc to turn it off
 	 */
 	set_opt(sbi->s_mount_opt, MBALLOC);