loops in get_user_pages() for VM_IO

[David S. Miller]

Some time recently, I don't know when, the logic in get_user_pages()
appears to have been changed a bit.

The inner-most loop of this routine basically does:

	while (follow_page() returns NULL)
		try handle_mm_fault();

The problem with this is that for !pfn_valid()
(for example, VM_IO areas mapping to I/O physical
address which have no assosciated page struct) follow_page()
will _always_ return NULL.

So when X tries to mmap() the frame buffer on my
system it loops forever here now.

Is pfn_valid() supposed to return true for I/O areas
too?  If so, where does the struct page backing store
come from?

It could be argued that setting VM_LOCKED is invalid.
And not setting it in drivers/video/sbuslib.c would make
this hang go away, but the above analysis means that
make_pages_present() cannot work on VM_IO areas.
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