[RFC] fs: filesystem specific options and remount

[Jerome Marchand <jmarchan-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

When remounted without option specified, some filesystems keep the
options that are already set (e.g. procfs, fat) and some reset them to
default (e.g. devpts).
Regarding options that are specified at remount, behavior of
filesystems also differ: some apply them (procfs, devpts), some
silently disregard them (e.g. fat) and some have a more elaborate
behavior (e.g. xfs apparently allows a subset of options to be changed
and issues warning if someone tries to change any other option).

Is there any policy regarding what the correct behavior should be?
This variety of behaviors tends to confuse mount utility which often
does not show the correct option actually set after a remount and
most certainly confuses the users as well.

Here is a example of discrepancy between mount (/etc/mtab) and
/proc/mounts:

$ grep proc /proc/mounts
/proc /proc proc rw,relatime 0 0
$ mount | grep proc
proc on /proc type proc (rw)

$ mount -o remount,hidepid=2 /proc/
$ grep proc /proc/mounts
/proc /proc proc rw,relatime,hidepid=2 0 0
$ mount | grep proc
proc on /proc type proc (rw,hidepid=2)

$ mount -o remount /proc/
$ grep proc /proc/mounts
/proc /proc proc rw,relatime,hidepid=2 0 0
$ mount | grep proc
proc on /proc type proc (rw)

And here is the discrepancy: mount does not show "hidepid=2" option
that is actually still set and enforced. Note that mount also missed
the relatime option to begin with.

Regards,
Jerome
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