i_writecount doubt

["Raghu R. Arur" <rra2002@cs.columbia.edu>]

182  if (file) {
183  struct inode *inode = file->f_dentry->d_inode;
184  get_file(file);
185  if (tmp->vm_flags & VM_DENYWRITE)
186  atomic_dec(&inode->i_writecount);
187

  I was looking at the code of dup_mmap() in fork.c. I didnt understand
something over here. The code above is checking whether the vm_area of the
parent that the new process (child process)is copying, has read-only
permission or read-write
permission. If it has read-only permission then the inode's i_writecount
is decremented. I saw in the vm documentation that if i_writecount is
negative then it is read-only and if it is positive then it is positive.
According to my understanding the inode data that we are accessing is
global. So some processes might have read-only access
to the file and some have read-write access to the file. So how can we
decide whether the process has the correct access just by seeing the value
of i_writecount of the inode. OR am i missing something over here.

  Can anyone please explain whats happening over here.

 thanks,
raghu

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