Re: [RFC][PATCH 00/27] Mount writer count and read-only bind mounts (v4)

[Ram Pai]

On Wed, Jul 12, 2006 at 11:17:09AM -0700, Dave Hansen wrote:
> Tries to incorporate comments from Al:
> http://article.gmane.org/gmane.linux.kernel/421029
> 
> Al wrote:
> >  b) figuring out what (if anything) should be done with
> >  propagation when we have shared subtrees... (not trivial at all)
> 
> Talked with Ram:  Shared subtrees are about having identical views
> into the filesystem.  Changing the mount permissions doesn't affect
> the view of the filesystem, so it should not be propagated.  


I think shared subtrees propagates mount/unmount events only.
This is a case where we are just changing the access permission
for a mount instance. So it should not be treated as a propagation event.

Lets say we propagated the event. In that case we would end up with a
awkward situation.

1) mount --make-shared /mnt
2) mount --bind /mnt /tmp
3) mount --make-slave /tmp
4) mount -o remount,rw /tmp
5) mount -o remount,ro /mnt

In step (5) all of a sudden the mount at /tmp which was readwrite will
be downgraded to read-only. There must have been a reason why /tmp was
mounted rw in the first place. Also there would be no way for /mnt to be
made 'ro' without effecting /tmp.

Hence I feel the readwrite semantics must be decoupled from the
sharedsubtree semantics,

RP


> 
> The things that probably need the heaviest review in here are the
> i_nlink monitoring patch (including the inode state flag patches 03
> and 06) and the new MNT_SB_WRITABLE flag (patch 05).  
> 
> ---
> 
> 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.  In this version, if there
> are writers on a superblock, the filesystem may not be remounted 
> r/o.  The same goes for MS_BIND mounts, and writers on a vfsmount.
> 
> This has a number of uses.  It allows chroots to have parts of
> filesystems writable.  It will be useful for containers in the future
> and is intended to replace patches that vserver has had out of the
> tree for several years.  It allows security enhancement by making
> sure that parts of your filesystem read-only, when you don't want
> to have entire new filesystems mounted, or when you want atime
> selectively updated.
> 
> This set 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.
> 
> Using this feature requires two steps:
> 
> 	mount --bind /source /dest
> 	mount -o remount,ro  /dest
> 
> Signed-off-by: Dave Hansen <haveblue@xxxxxxxxxx>
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