ext4: do not set extents feature from the kernel

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.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 17 insertions, 4 deletions

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 2bf9cdd7a036..1cb371dcd609 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -1324,6 +1324,13 @@ 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:
@@ -1997,12 +2004,18 @@ static int ext4_fill_super (struct super_block *sb, void *data, int silent)
 
         /*
          * 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);