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mm: adjust page migration heuristic
The page allocator's heuristic to decide when to migrate page blocks to unmovable seems to have been tuned on architectures that do not have kernel drivers that would make unmovable allocations of several megabytes or greater--ie, no cameras or shared-memory GPUs. The number of allocations from these drivers may be unbounded and may occupy a significant percentage of overall system memory (>50%). As a result, every Android device has suffered to some extent from increasing fragmentation due to unmovable page block migration over time. This change adjusts the page migration heuristic to only migrate page blocks for unmovable allocations when the order of the requested allocation is order-5 or greater. This prevents migration due to GPU and ion allocations so long as kernel drivers allocate memory at runtime using order-4 or smaller pages. Experimental results running the Android longevity test suite on a Nexus 5X for 10 hours: old heuristic: 116 unmovable blocks after boot -> 281 unmovable blocks new heuristic: 105 unmovable blocks after boot -> 101 unmovable blocks bug 26916944 Change-Id: I5b7ccbbafa4049a2f47f399df4cb4779689f4c40
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