mm: change lookup_node() to use get_user_pages_fast()

The purpose of calling get_user_pages_locked() from lookup_node() was to
allow for unlocking the mmap_lock when reading a page from the disk
during a page fault (hidden behind VM_FAULT_RETRY).  The idea was to
reduce contention on the heavily-used mmap_lock.  (Thanks to Jan Kara
for clearly pointing that out, and in fact I've used some of his wording
here.)

However, it is unlikely for lookup_node() to take a page fault.  With
that in mind, change over to calling get_user_pages_fast().  This
simplifies the code, runs a little faster in the expected case, and
allows removing get_user_pages_locked() entirely, in a subsequent patch.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: John Hubbard <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Reviewed-by: Jason Gunthorpe <[email protected]>
Reviewed-by: Claudio Imbrenda <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Cc: Alex Williamson <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Lukas Bulwahn <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Peter Xu <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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