Re: ext4 ignoring rootfs default mount options

[lsorense@csclub.uwaterloo.ca (Lennart Sorensen)]

On Wed, Mar 07, 2018 at 11:08:56AM -0500, Theodore Y. Ts'o wrote:
> This is where it's critcal to understand that the "tune2fs -o" changes
> the *default* mount options.  The key in the comment is the anything
> different from the *filesystem* defaults (that is, the defaults of the
> particular ext4 file system, as opposed to the global defaults).  The
> idea is that /proc/mounts, and /etc/mtab shows the options string that
> if used in /etc/fstab, or in the mount command line, will replicate
> the current mount option state.  Furthermore, that /proc/mounts is the
> minimal set of mount option strings.
> 
> You may not like the behavior, but it's been this way forever, and the
> reasoning behind it is that the low-level file system code doesn't
> really know what the actual mount option string that would be in
> /etc/fstab or in the /sbin/mount command line.  That's because
> /sbin/mount command actually parses the mount options, and things like
> "ro" is actually translated into bitflag passed to the mount option.
> So for example, it's impossible to know whether "rw" was in the
> user-specified mount string, since we never see it by the time the
> string gets sent to ext4_fill_super (in fact the kernel never sees
> it).  So when we try to make /proc/mounts a replacement for /etc/mtab
> (since some people make /etc/mtab as symlink /proc/mounts), it's
> actually impossible.  Trying to make it be the minimal set of options
> was at least a consitent thing.  That is, if you use "tune2fs -o
> nodelalloc", it's not necessary to put nodelalloc in /etc/fstab or in
> the mount command line.  And hence, it should not be in /proc/mounts.

OK, that makes sense.  Thanks.  I will work on convincing myself this
is how it should be.

> As far as where ext4_seq_options_show() gets called, it's because we
> have a fair amount of macro shortcuts in fs/ext4/sysfs.c (which is
> where we put all of the pseudo file system support for ext4, which
> means it includes procfs).  Search for macros PROC_FILE_SHOW_DEFN and
> PROC_FILE_LIST.

Oh that's where it is.

-- 
Len Sorensen