Re: ext4 ignoring rootfs default mount options

[Theodore Y. Ts'o]

On Tue, Mar 06, 2018 at 02:03:15PM -0500, Lennart Sorensen wrote:
> While switching a system from using ext3 to ext4 (It's about time) I
> discovered that setting default options for the filesystem using tune2fs
> -o doesn't work for the root filesystem when mounted by the kernel itself.
> Filesystems mounted from userspace with the mount command use the options
> set just fine.  The extended option set with tune2fs -E mount_opts=
> works fine however.

Well.... it's not that it's being ignored.  It's just a
misunderstanding of how a few things.  It's also that the how we
handled mount options has evolved over time, leading to a situation
which is confusing.

First, tune2fs changes the default of ext4's mount options.  This is
stated in the tune2fs man page:

-o [^]mount-option[,...]
              Set or clear the indicated default mount options in the filesysâ
              tem.   Default  mount options can be overridden by mount options
              specified either in /etc/fstab(5) or on the command  line  arguâ
              ments  to mount(8).  Older kernels may not support this feature;
              in particular, kernels which predate  2.4.20  will  almost  cerâ
              tainly ignore the default mount options field in the superblock.

Secondly, the message when af ile sytem is mounted, e.g.:

> EXT4-fs (sda1): mounted filesystem with ordered data mode. Opts: (null)

... is the mount option string that are passed to the mount system
call.

The extended mount options is different.  It was something that we
added later.  If it is present, this the extended mount options is
printed first, followed by a semi-colon, followed by string passed to
the mount system call.

Hence:

> tune2fs -E mount_opts=nodelalloc /dev/sda1
> 
> at boot we got:
> EXT4-fs (sda1): mounted filesystem with ordered data mode. Opts: nodelalloc; (null)


The description of -E option in the tune2fs man page talks about some
of this, but it's arguably confusing.

You can see exactly what mount options that are active by looking at
the file /proc/fs/ext4/<dev>/options.  So this is how you can prove to
yourself that tune2fs -o works.

root@kvm-xfstests:~# dmesg -n 7
root@kvm-xfstests:~# tune2fs -o nodelalloc /dev/vdc 
tune2fs 1.44-WIP (06-Sep-2017)
root@kvm-xfstests:~# mount /dev/vdc /vdc
[   27.389192] EXT4-fs (vdc): mounted filesystem with ordered data mode. Opts: (null)
root@kvm-xfstests:~# cat /proc/fs/ext4/vdc/options 
rw
bsddf
nogrpid
block_validity
dioread_lock
nodiscard
nodelalloc
journal_checksum
barrier
auto_da_alloc
user_xattr
acl
noquota
resuid=0
resgid=0
errors=continue
commit=5
min_batch_time=0
max_batch_time=15000
stripe=0
data=ordered
inode_readahead_blks=32
init_itable=10
max_dir_size_kb=0

> For filesystems mounted from userspace with the mount command, either
> method works however.  The first option however is what the comment in
> fs/ext4/super.c suggests to use.
> 
> Of course I also got the messages:
> EXT4-fs (sda1): Mount option "nodelalloc" incompatible with ext3
> EXT4-fs (sda1): failed to parse options in superblock: nodelalloc
> EXT4-fs (sda1): couldn't mount as ext3 due to feature incompatibilities

So what's happening here is something that has recently started
getting reported by users.  Most modern distro's use an initial
ramdisk to mount the root file system, and they use blkid to determine
the file system with the right file system type.  If the kernel is
mounting the root file system.  An indication that this is what's
happening is the following message in dmesg:

[    2.196149] VFS: Mounted root (ext4 filesystem) readonly on device 254:0.

This message means the kernel fallback code was used to mount the file
system, not the initial ramdisk code in userspace.

If you are using the kernel fallback code, it will first try to mount
the file system as ext3, and if you have "nodelalloc" in the extended
mount options in the superblock, it will try it first.  The messages
you have quoted above are harmless.  But they are scaring users, so we
are looking into ways to suppress them.

> And of course the last annoying thing I noticed is that /proc/mounts
> doesn't actually tell you that nodelalloc is active when it is set
> from the default mount options rather than from the mount command line
> (or fstab).  Lots of other non default options are explicitly handled,
> but not delalloc.  The only place you see it, is in the dmesg line
> telling you what options the filesystem was mounted with.

That's because /proc/mounts is trying to emulate the user-space
maintained /etc/mtab file.  So we deliberately suppress default mount
options.  If you take out this feature:

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 756f515b762d..e93b86f68da5 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -2038,8 +2038,8 @@ static int _ext4_show_options(struct seq_file *seq, struct super_block *sb,
 		if (((m->flags & (MOPT_SET|MOPT_CLEAR)) == 0) ||
 		    (m->flags & MOPT_CLEAR_ERR))
 			continue;
-		if (!(m->mount_opt & (sbi->s_mount_opt ^ def_mount_opt)))
-			continue; /* skip if same as the default */
+//		if (!(m->mount_opt & (sbi->s_mount_opt ^ def_mount_opt)))
+//			continue; /* skip if same as the default */
 		if ((want_set &&
 		     (sbi->s_mount_opt & m->mount_opt) != m->mount_opt) ||
 		    (!want_set && (sbi->s_mount_opt & m->mount_opt)))


... then /proc/mounts looks a lot messier, and most users would not
like the result:

/dev/vdc /vdc ext4 rw,relatime,bsddf,nogrpid,block_validity,dioread_lock,nodiscard,nodelalloc,journal_checksum,barrier,auto_da_alloc,user_xattr,acl,noquota,data=ordered 0 0

If you really want the reliable "what are the mount options right
now", the place to look is /proc/fs/ext4/<device>/options, as
described above.

Cheers,

							- Ted