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

[Mike Waychison <Michael.Waychison@Sun.COM>]

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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?

- --
Mike Waychison
Sun Microsystems, Inc.
1 (650) 352-5299 voice
1 (416) 202-8336 voice

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