Re: [PATCH 13/28] VFS: Introduce soft reference counts

[Mika Penttilä]

Mike Waychison wrote:

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> Mika Penttilä wrote:
>  
>
> > Mike Waychison wrote:
> >
> >    
> >
> > > This patch introduces the concept of a 'soft' reference count for a
> > > vfsmount.
> > > This type of reference count allows for references to be held on
> > > mountpoints
> > > that do not affect their busy states for userland unmounting.  Some might
> > > argue that this is wrong because 'when I unmount a filesystem, I want the
> > > resources associated with it to go away too', but this way of thinking
> > > was
> > > deprecated with the addition of namespaces and --bind back in the 2.4
> > > series.
> > >
> > > A future addition may see a callback mechanism so that in kernel users
> > > can
> > > use a given mountpoint and have it deregistered some way (quota and
> > > accounting come to mind).
> > >
> > > These soft reference counts are used by a later patch that adds an
> > > interface
> > > for holding and manipulating mountpoints using filedescriptors.
> > >
> > > Signed-off-by: Mike Waychison <michael.waychison@xxxxxxx>
> > >
> > > +static inline struct vfsmount *mntsoftget(struct vfsmount *mnt)
> > > +{
> > > +    if (mnt) {
> > > +        read_lock(&vfsmountref_lock);
> > > +        atomic_inc(&mnt->mnt_softcount);
> > > +        mntgroupget(mnt);
> > > +        read_unlock(&vfsmountref_lock);
> > > +    }
> > > +    return mnt;
> > > +}
> > > +
> > > +static inline void mntsoftput(struct vfsmount *mnt)
> > > +{
> > > +    struct vfsmount *cleanup;
> > > +    might_sleep();
> > > +    if (mnt) {
> > > +        if (atomic_dec_and_test(&mnt->mnt_count))
> > > +            __mntput(mnt);
> > > +        read_lock(&vfsmountref_lock);
> > > +        cleanup = mntgroupput(mnt);
> > > +        atomic_dec(&mnt->mnt_softcount);
> > > +        read_unlock(&vfsmountref_lock);
> > > +        if (cleanup)
> > > +            __mntgroupput(cleanup);
> > > +    }
> > > +}
> > > +
> > > extern void free_vfsmnt(struct vfsmount *mnt);
> > >
> > >
> > >      
> > What is this against? What are mntgroupput and mntgroupget? 
> >    
>
> This is against patch [PATCH 11/28] VFS: Allow detachable subtrees.
>
> In that patch, mntgroup(get|put) handles the count of all non-glue
> references for a given tree of vfsmounts.
>
>
>  
>
> > Why does soft put decrement mnt_count which isn't increment by soft get? 
> >    
>
> Ah, thanks for pointing that out.  It got messed up when I created the
> patchset from the bk tree. Will fix.
>
>  
>
> > How do
> > soft references allow userland umount? I don't see soft references used
> > anywhere...
> >    
>
> Soft references are used by the mountpoint file descriptor patch
> [14/28].  They allow references to be had on a vfsmount such that the
> mountpoint itself is not kept busy in the namespace.  This allows a
> program to 'grab a mountpoint' by a magic file (gotten by sys_mountfd),
> and perform ops on it.   The mountfd holds a reference to the vfsmount,
> but it doesn't keep userspace from trying to umount(2) the path.
>
> Does that help?
>
>  
I think at least patches 11 and 14 got lost...

--Mika



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