Re: [PATCH 00/20] Mount writer count and read-only bind mounts (v3)

[Andrew Morton <akpm@osdl.org>]

On Tue, 27 Jun 2006 15:14:36 -0700
Dave Hansen <haveblue@us.ibm.com> wrote:

> I think these are ready for -mm.  They've gone through a few revisions
> and a week or two of normal burn-in testing.  
> 
> ---
> 
> The following series implements read-only bind mounts.  This feature
> allows a read-only view into a read-write filesystem.  In the process
> of doing that, it also provides infrastructure for keeping track of
> the number of writers to any given mount.  New in this version is that
> if that number is non-zero, remounts from r/w to r/o are not allowed.  
> 
> This set does not take the previously tried approach of pushing down
> the vfsmount structure deeply into call paths, such that it might be
> checked in functions like permission(), may_create() and may_open().
> Instead, it does checks near the entry points in the kernel, bumping
> a reference count in the vfsmount structure.  I've also eliminated
> the use of the MNT_RDONLY flag.  It was redundant since we have the
> reference count.
> 
> This set also makes no attempt to keep the return codes for these
> r/o bind mounts the same as for a real r/o filesystem or device.
> It would require significantly more code and be quite a bit more
> invasive.  Unless there is a very strong reason to do so, I believe
> it isn't worth the trouble.
> 
> One note: the previous patches all worked this way:
> 
> 	mount --bind -o ro /source /dest
> 
> These patches have changed that behavior.  It now requires two steps:
> 
> 	mount --bind /source /dest
> 	mount -o remount,ro  /dest

That seems a step backwards.

> Since the last revision, the locking in faccessat() and
> mnt_is_readonly() has been changed to fix a race which might have
> caused a false-negative mount-is-readonly return when faccessat()
> is called while another two processes are racing to make a mount
> readonly.
> 
> Signed-off-by: Dave Hansen <haveblue@us.ibm.com>

umm, what's it all for?