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

[Ram Pai <linuxram@us.ibm.com>]

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?

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





> 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.

> +/*
> + * 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)

RP
> +
> 
>  struct vfsmount {
>  	struct list_head mnt_hash;
-- 
Ram Pai
System X Device-Driver Enablement Lead
Linux Technology Center
Beaverton OR-97006
503-5783752 t/l 7753752
linuxram@us.ibm.com