Date: Fri, 23 Feb 2007 07:46:32 +0100 (CET) From: "Tobias Roth" <ports@fsck.ch> To: freebsd-current@freebsd.org, freebsd-fs@freebsd.org Subject: Re: What does "mount -o union" and MNT_UNION really do? Message-ID: <1238.62.2.233.2.1172213192.squirrel@secure.socket.ch> In-Reply-To: <20070222222301.GA13464@crodrigues.org> References: <20070222222301.GA13464@crodrigues.org>
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[resend, as I forgot to cc the lists and only replied to Craig] On Thu, February 22, 2007 11:23 pm, Craig Rodrigues wrote: > Hi, > > As part of recent cleanups and stability fixes in vfs_syscalls.c, > Kostik Belousov removed the union_dircheckp() callback > which the old unionfs implementation used, but the new one does not. > > Now I am looking at the mount code in vfs_mount.c, and > am trying to figure out what the MNT_UNION flag > is used for, which is set by doing "mount -o union". > The following code in vfs_mount.c sets it: > > else if (strcmp(opt->name, "union") == 0) > fsflags |= MNT_UNION; > > > However, the mount_unionfs binary never passes down "-o union", > so the MNT_UNION flag is never set if you do: "mount -t unionfs" > or "mount_unionfs". > > The mount(8) man page documents it as: > union Causes the namespace at the mount point to appear as > the > union of the mounted file system root and the > existing > directory. Lookups will be done in the mounted file > sys- > tem first. If those operations fail due to a > non-exis- > tent file the underlying directory is then accessed. > All > creates are done in the mounted file system. > > > Is there a legitimate case where you would > want to do "mount -o union", and have it behave differently > from "mount_unionfs / mount -t unionfs"? Or is this a leftover from > a long time ago that we can now whack (it would simplify some code > in the VFS layer if we whack it)? > > The MNT_UNION flag seemed to appear a long time ago: > > revision 1.120 > date: 1999/03/03 02:35:51; author: julian; state: Exp; lines: +35 -33 > Slight cleanup of code resurected for union mounts.. > Submitted by: Tony Finch <dot@dotat.at> > ---------------------------- > revision 1.119 > date: 1999/02/27 07:06:05; author: julian; state: Exp; lines: +23 -1 > Fix code for union mounts > Accidentally deleted by peter when he extracted the unionfs stuff in 1.109 > > Submitted by: Tony Finch <dot@dotat.at> > ---------------------------- > revision 1.109 > date: 1998/11/03 08:01:47; author: peter; state: Exp; lines: +15 -114 > Change the #ifdef UNION code into a callable hook. Arrange to have this > set up when unionfs is present, either statically or as a kld module. > > > -- > Craig Rodrigues > rodrigc@crodrigues.org > _______________________________________________ > freebsd-current@freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-current > To unsubscribe, send any mail to "freebsd-current-unsubscribe@freebsd.org" >
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