We've been getting warnings about an excessive amount of time spent
allocating pages for migration during memory compaction without
scheduling.  isolate_freepages_block() already periodically checks for
contended locks or the need to schedule, but isolate_freepages() never
does.

When a zone is massively long and no suitable targets can be found, this
iteration can be quite expensive without ever doing cond_resched().

Check periodically for the need to reschedule while the compaction free
scanner iterates.
Signed-off-by: NDavid Rientjes <rientjes@google.com>
Reviewed-by: NRik van Riel <riel@redhat.com>
Reviewed-by: NWanpeng Li <liwanp@linux.vnet.ibm.com>
Acked-by: NMel Gorman <mgorman@suse.de>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>