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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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