mm, page_alloc: fallback to smallest page when not stealing whole pageblock
Since commit 3bc48f96 ("mm, page_alloc: split smallest stolen page in fallback") we pick the smallest (but sufficient) page of all that have been stolen from a pageblock of different migratetype. However, there are cases when we decide not to steal the whole pageblock. Practically in the current implementation it means that we are trying to fallback for a MIGRATE_MOVABLE allocation of order X, go through the freelists from MAX_ORDER-1 down to X, and find free page of order Y. If Y is less than pageblock_order / 2, we decide not to steal all pages from the pageblock. When Y > X, it means we are potentially splitting a larger page than we need, as there might be other pages of order Z, where X <= Z < Y. Since Y is already too small to steal whole pageblock, picking smallest available Z will result in the same decision and we avoid splitting a higher-order page in a MIGRATE_UNMOVABLE or MIGRATE_RECLAIMABLE pageblock. This patch therefore changes the fallback algorithm so that in the situation described above, we switch the fallback search strategy to go from order X upwards to find the smallest suitable fallback. In theory there shouldn't be a downside of this change wrt fragmentation. This has been tested with mmtests' stress-highalloc performing GFP_KERNEL order-4 allocations, here is the relevant extfrag tracepoint statistics: 4.12.0-rc2 4.12.0-rc2 1-kernel4 2-kernel4 Page alloc extfrag event 25640976 69680977 Extfrag fragmenting 25621086 69661364 Extfrag fragmenting for unmovable 74409 73204 Extfrag fragmenting unmovable placed with movable 69003 67684 Extfrag fragmenting unmovable placed with reclaim. 5406 5520 Extfrag fragmenting for reclaimable 6398 8467 Extfrag fragmenting reclaimable placed with movable 869 884 Extfrag fragmenting reclaimable placed with unmov. 5529 7583 Extfrag fragmenting for movable 25540279 69579693 Since we force movable allocations to steal the smallest available page (which we then practially always split), we steal less per fallback, so the number of fallbacks increases and steals potentially happen from different pageblocks. This is however not an issue for movable pages that can be compacted. Importantly, the "unmovable placed with movable" statistics is lower, which is the result of less fragmentation in the unmovable pageblocks. The effect on reclaimable allocation is a bit unclear. Link: http://lkml.kernel.org/r/20170529093947.22618-1-vbabka@suse.czSigned-off-by: NVlastimil Babka <vbabka@suse.cz> Acked-by: NMel Gorman <mgorman@techsingularity.net> Cc: Johannes Weiner <hannes@cmpxchg.org> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com> Cc: David Rientjes <rientjes@google.com> Signed-off-by: NAndrew Morton <akpm@linux-foundation.org> Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
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