That is exactly what you cannot do. Otherwise you have to rewrite
filesystems not to use the pagecache, and probably have to do disallow
or do some horrible hacks to support mmap.
The requirement you actually intend is more like what you said before,
along the lines of "file can't be changed in given set of circumstances".
It is a best effort thing. When you start mmapping, that allows
get_user_pages, which in turn may be able to take pages and modify
them at fairly arbitrary points in time.
Even just with mmap (we do a much better job of it now, but) you
can probably write to a file after mtime if you write to already
dirty pages.
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