unionfs requirement for tmpfs

[Howard Su]

Hi All,

I am working on tmpfs to make it support unionfs. I think I understand the
theory of the unionfs. However I want someone can give me some info
regarding the implementation details, like ISWHITEOUT, DOWHITEOUT op.

As far as I know, a fs can be used as upper level and lower level fs of a
unionfs. The requirement should be different:

1. As a upper level (which will be changed. I am more interested to make
tmpfs support this)
what is the requirement? So far, based on my code reading on ufs/unionfs, I
need implement VOP_WHITEOUT with three OPs: LOOKUP, DELETE, CREATE. what'
the expctation of input and output, the code reading can not give me a clear
answer. ufs use some hacking way to call into ufs code itself to make the
logic unclear to me.

2. As a lower level fs.
what's the requirement? I have no idea totally.
Share me whatever you know, please.

-- 
-Howard