When unmapping N pages (e.g. shared memory) the amount of TLB flushes
done can be (N*PAGE_SIZE/ZAP_BLOCK_SIZE)*N although it should be N at
maximum. With PREEMPT kernel ZAP_BLOCK_SIZE is 8 pages, so there is a
noticeable performance penalty when unmapping a large VMA and the system
is spending its time in flush_tlb_range().

The problem is that tlb_end_vma() is always flushing the full VMA
range. The subrange that needs to be flushed can be calculated by
tlb_remove_tlb_entry(). This approach was suggested by Hugh Dickins,
and is also used by other arches.

The speed increase is roughly 3x for 8M mappings and for larger mappings
even more.
Signed-off-by: NAaro Koskinen <Aaro.Koskinen@nokia.com>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>