Re: [PATCH] VFS: Clean up shared mount flag propagation

[Valerie Aurora <vaurora@redhat.com>]

On Fri, Jan 29, 2010 at 09:52:17AM -0800, Ram Pai wrote:
> On Tue, 2010-01-26 at 14:20 -0500, Valerie Aurora wrote:
> > Al, you might have already fixed this as part of your mount flag
> > rewrite, but here's a patch to clean up some problems with shared
> > mount flag propagation that I ran into while working on union mounts.
> > Against 2.6.32.
> > 
> > -VAL
> > 
> > commit 3f436c4466f278900d451494b317f6bb80b4c24b
> > Author: Valerie Aurora <vaurora@redhat.com>
> > Date:   Tue Jan 26 11:09:00 2010 -0800
> > 
> >     VFS: Clean up shared mount flag propagation
> >     
> >     The handling of mount flags in set_mnt_shared() got a little tangled
> >     up during previous cleanups, with the following problems:
> >     
> >     * MNT_PNODE_MASK is defined as a literal constant when it should be a
> >       bitwise xor of other MNT_* flags
> >     * set_mnt_shared() clears and then sets MNT_SHARED (part of MNT_PNODE_MASK)
> >     * MNT_PNODE_MASK could use a comment in mount.h
> >     * MNT_PNODE_MASK is a terrible name, change to MNT_SHARED_MASK
> >     
> >     This patch fixes these problems.
> > 
> > diff --git a/fs/pnode.h b/fs/pnode.h
> > index 958665d..adc52ca 100644
> > --- a/fs/pnode.h
> > +++ b/fs/pnode.h
> > @@ -26,7 +26,7 @@
> > 
> >  static inline void set_mnt_shared(struct vfsmount *mnt)
> >  {
> > -	mnt->mnt_flags &= ~MNT_PNODE_MASK;
> > +	mnt->mnt_flags &= ~MNT_SHARED_MASK;
> >  	mnt->mnt_flags |= MNT_SHARED;
> >  }
> > 
> 
> Is resetting the MNT_SHARED bit and setting it back, causing a issue?

What was confusing is defining MNT_PNODE_MASK to remove the MNT_SHARED
bit, and then having exactly one use in the source base which then
immediately set it back.  That looks like a bug.  However, it looks
like the real bug was not using MNT_PNODE_MASK to save these bits in
do_remount(), fixed in commit
7b43a79f32c0a05e7562043af98e25c05c89b18e.

> If so, why not just do
>   -      mnt->mnt_flags &=  ~MNT_PNODE_MASK;
>   +      mnt->mnt_flags &=  ~MNT_UNBINDABLE; 

I think this is much clearer and easier to understand.

> > diff --git a/include/linux/mount.h b/include/linux/mount.h
> > index 5d52753..257985f 100644
> > --- a/include/linux/mount.h
> > +++ b/include/linux/mount.h
> > @@ -34,7 +34,15 @@ struct mnt_namespace;
> > 
> >  #define MNT_SHARED	0x1000	/* if the vfsmount is a shared mount */
> >  #define MNT_UNBINDABLE	0x2000	/* if the vfsmount is a unbindable mount */
> > -#define MNT_PNODE_MASK	0x3000	/* propagation flag mask */
> 
> I prefer to leave the name MNT_PNODE_MASK as is, because it captures the
> notion of propagation v/s the notion of just being shared.

I understand that this name is obvious to you as the author of this
code, but it doesn't convey any useful information to me. :) Perhaps:

/*
 * We propogate most mnt flag changes to peer mounts, but not the
 * flags that specify whether or not propogation occurs.
 */
#define MNT_PROPOGATION_MASK (MNT_UNBINDABLE | MNT_SHARED)

> > +/*
> > + * MNT_SHARED_MASK is the set of flags that should be cleared when a
> > + * mount becomes shared.  Currently, this is only the flag that says a
> > + * mount cannot be bind mounted, since this is how we create a mount
> > + * that shares events with another mount.  If you add a new MNT_*
> > + * flag, consider how it interacts with shared mounts.
> > + */
> > +#define MNT_SHARED_MASK	(MNT_UNBINDABLE)
> 
> 
> and make this
>   +#define MNT_SHARED_MASK        (MNT_UNBINDABLE | MNT_SHARED)

Agree with this.

-VAL