Re: [PATCH] nfs: Do not allow multiple mounts on same mountpoint when using -o noac
On Mon, 2011-08-01 at 07:19 -0400, Christoph Hellwig wrote:
> On Mon, Aug 01, 2011 at 12:10:12PM +0100, Sachin Prabhu wrote:
> > Do not allow multiple mounts on same mountpoint when using -o noac
> The patch content really doesn't seem to match the subject line and
> most of the content.
> > However when using the option 'noac', the user is able to mount the same
> > share on the same mountpoint multiple times. This happens because a
> > share mounted with the noac option is automatically assigned the 'sync'
> > flag MS_SYNCHRONOUS in nfs_initialise_sb(). This flag is set after the
> > check for already existing superblocks is done in sget(). The check for
> > the mount flags in nfs_compare_mount_options() does not take into
> > account the 'sync' flag applied later on in the code path. This means
> > that when using 'noac', a new superblock structure is assigned for every
> > new mount of the same share and multiple shares on the same mountpoint
> > are allowed.
> > ie.
> > # mount -onoac localhost:/nfsv3 /mnt
> > can be run multiple times.
> > The patch checks for noac and assigns the sync flag before sget() is
> > called to obtain an already existing superblock structure.
> That's a fine fix, but the the whole patch subject and description
> focusses on a side effect rather than the underlying real problem.
> The underlying issue is that youwant to share superblocks when
> having multiple mounts with -o noac, which requires assigning the