mm/compaction: make isolate_freepages start at pageblock boundary

mm/compaction: make isolate_freepages start at pageblock boundary

The compaction freepage scanner implementation in isolate_freepages()
starts by taking the current cc->free_pfn value as the first pfn.  In a
for loop, it scans from this first pfn to the end of the pageblock, and
then subtracts pageblock_nr_pages from the first pfn to obtain the first
pfn for the next for loop iteration.

This means that when cc->free_pfn starts at offset X rather than being
aligned on pageblock boundary, the scanner will start at offset X in all
scanned pageblock, ignoring potentially many free pages.  Currently this
can happen when

 a) zone's end pfn is not pageblock aligned, or

 b) through zone->compact_cached_free_pfn with CONFIG_HOLES_IN_ZONE
    enabled and a hole spanning the beginning of a pageblock

This patch fixes the problem by aligning the initial pfn in
isolate_freepages() to pageblock boundary.  This also permits replacing
the end-of-pageblock alignment within the for loop with a simple
pageblock_nr_pages increment.

Signed-off-by: Vlastimil Babka <[email protected]>
Reported-by: Heesub Shin <[email protected]>
Acked-by: Minchan Kim <[email protected]>
Cc: Mel Gorman <[email protected]>
Acked-by: Joonsoo Kim <[email protected]>
Cc: Bartlomiej Zolnierkiewicz <[email protected]>
Cc: Michal Nazarewicz <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Christoph Lameter <[email protected]>
Acked-by: Rik van Riel <[email protected]>
Cc: Dongjun Shin <[email protected]>
Cc: Sunghwan Yun <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>