Approval request of some additions to src/sys/kern/vfs_subr.c and src/sys/sys/vnode.h

[Daichi GOTO]

Kostik Belousov wrote:
> On Fri, Apr 25, 2008 at 07:21:20PM +0900, Daichi GOTO wrote:
>> Hi Konstantin :)
>>
>> To fix a unionfs issue of http://www.freebsd.org/cgi/query-pr.cgi?pr=109377,
>> we need to add new functions
>>
>>    void vkernrele(struct vnode *vp);
>>    void vkernref(struct vnode *vp);
>>
>> and one value
>>
>>    int	v_kernusecount; /* i ref count of kernel */
>>
>> to src/sys/sys/vnode.h and rc/sys/kern/vfs_subr.c.
>>
>> Unionfs will be panic when lower fs layer is forced umounted by
>> "umount -f".  So to avoid this issue, we've added
>> "v_kernusecount" value that means "a vnode count that kernel are
>> using".  vkernrele() and vkernref() are functions that manage
>> "v_kernusecount" value.
>>
>> Please check those and give us an approve or some comments!
> 
> There is already the vnode reference count. From your description, I cannot
> understand how the kernusecount would prevent the panic when forced unmount
> is performed. Could you, please, show the actual code ? PR mentioned
> does not contain any patch.

Our patch realizes avoiding kernel panic by "umount -f" operation using with
EBUSY process.

On current implementation (not applied our patch), "umount -f" tries to
release vnode at any vnode reference count value. Since that, unionfs
and nullfs access invalid vnode and lead kernel panic. To prevent this
issue, we need a some kind of not-umount-accept-mechanism in invalid case
(e.x. fs in unionfsed stack, it must be umounted in correct order) and
to realize that, current vnode reference count is not enough we are thinking.

If you have any ideas to realize the same solution with current vnode
reference, would you please tell us your idea :)

> The problem you described is common for the kernel code, and right way
> to handle it, for now, is to keep refcount _and_ check for the forced
> reclaim.

-- 
   Daichi GOTO, http://people.freebsd.org/~daichi