mm, compaction: remember position within pageblock in free pages scanner

Unlike the migration scanner, the free scanner remembers the beginning of
the last scanned pageblock in cc->free_pfn.  It might be therefore
rescanning pages uselessly when called several times during single
compaction.  This might have been useful when pages were returned to the
buddy allocator after a failed migration, but this is no longer the case.

This patch changes the meaning of cc->free_pfn so that if it points to a
middle of a pageblock, that pageblock is scanned only from cc->free_pfn to
the end.  isolate_freepages_block() will record the pfn of the last page
it looked at, which is then used to update cc->free_pfn.

In the mmtests stress-highalloc benchmark, this has resulted in lowering
the ratio between pages scanned by both scanners, from 2.5 free pages per
migrate page, to 2.25 free pages per migrate page, without affecting
success rates.

With __GFP_NO_KSWAPD allocations, this appears to result in a worse ratio
(2.1 instead of 1.8), but page migration successes increased by 10%, so
this could mean that more useful work can be done until need_resched()
aborts this kind of compaction.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Acked-by: David Rientjes <rientjes@google.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Acked-by: Mel Gorman <mgorman@suse.de>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Michal Nazarewicz <mina86@mina86.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>