Identify tasks which mmap'ed file

Within the kernel and given a struct file *, is it possible to enumerate the tasks which have mmap/MAP_SHARED'ed the file? I have tried to use find_get_pages/rmap_walk on the file's f_mapping field, but this does not seem to work. I find the mapping's first page with find_get_pages, but rmap_walk does not seem to find any vma's as rmap_walk never calls the callback function I pass in. Thank you, -- Mike :wq