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

["Mika Penttilä" <mika.penttila@kolumbus.fi>]

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@sun.com>
>>>
>>>+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