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Subjectbug with multiple mounts of filesystems in 2.6

Hi --

We're working on migrating to the 2.6 kernel series, and one big
problem has popped up: we have a number of NFS mounts that are
mounted read-only in one location and read-write in a distinct
location (on the same machine). With 2.4 series kernels, this worked
without issue, but with 2.6, it doesn't: it's not possible to mount
the same filesystem twice with different options for each mount; the
two mount points have to share the same mount options.

Furthermore, if you do mount one filesystem at two different places,
changing the mount options on one mount point ('mount -o remount,rw
$MOUNT1', for example) also results in the mount options on the
other mount point being changed. Finally, the information returned
by 'mount' about the mount points is wrong -- 'mount' will show (for
example) one mount point being 'rw' and the other being 'ro', when
in fact attempting to use the 'rw' mount point will show it to be
read-only. /proc/mounts correctly indicates that both mounts are

Further experimentation has shown:

* the NFS layer is not involved; it is possible to reproduce this
problem using just locally-attached filesystems.
* this affects at least 2.6.3, 2.6.5, and 2.6.7.
* it happens with mount version 2.11r and 2.12.

Because the options of the first mount point "wins" (i.e., mounting a
filesystem once read-write, and then a second time as read-only,
leaves both mounts in a read-write state), it seems like there's
some sort of caching optimization going on -- but I haven't looked
into the code to find out if that guess is correct.

Any pointers towards restoring the 2.4 behavior welcomed.

Perhaps you have forgotten that this is an engineering discipline, not
some sort of black magic.
Mark Jason Dominus's Good Advice #11946
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