mm, compaction: use proper alloc_flags in __compaction_suitable()

mm, compaction: use proper alloc_flags in __compaction_suitable()

The __compaction_suitable() function checks the low watermark plus a
compact_gap() gap to decide if there's enough free memory to perform
compaction.  This check uses direct compactor's alloc_flags, but that's
wrong, since these flags are not applicable for freepage isolation.

For example, alloc_flags may indicate access to memory reserves, making
compaction proceed, and then fail watermark check during the isolation.

A similar problem exists for ALLOC_CMA, which may be part of alloc_flags,
but not during freepage isolation.  In this case however it makes sense to
use ALLOC_CMA both in __compaction_suitable() and __isolate_free_page(),
since there's actually nothing preventing the freepage scanner to isolate
from CMA pageblocks, with the assumption that a page that could be
migrated once by compaction can be migrated also later by CMA allocation.
Thus we should count pages in CMA pageblocks when considering compaction
suitability and when isolating freepages.

To sum up, this patch should remove some false positives from
__compaction_suitable(), and allow compaction to proceed when free pages
required for compaction reside in the CMA pageblocks.

Link: http://lkml.kernel.org/r/20160810091226.6709-10-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>