mm-highmem-makes-flush_all_zero_pkmaps-return-index-of-last-flushed-entry-v2

mm-highmem-makes-flush_all_zero_pkmaps-return-index-of-last-flushed-entry-v2

In current code, after flush_all_zero_pkmaps() is invoked,
then re-iterate all pkmaps. It can be optimized if flush_all_zero_pkmaps()
return index of first flushed entry. With this index,
we can immediately map highmem page to virtual address represented by index.
So change return type of flush_all_zero_pkmaps()
and return index of first flushed entry.

Additionally, update last_pkmap_nr to this index.
It is certain that entry which is below this index is occupied by other mapping,
therefore updating last_pkmap_nr to this index is reasonable optimization.

Signed-off-by: Joonsoo Kim <js1304@gmail.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>