[PATCH] use compound pages for hugetlb pages only
The compound page logic is a little fragile - it relies on additional metadata in the pageframes which some other kernel code likes to stomp on (xfs was doing this). Also, because we're treating all higher-order pages as compound pages it is no longer possible to free individual lower-order pages from the middle of higher-order pages. At least one ARM driver insists on doing this. We only really need the compound page logic for higher-order pages which can be mapped into user pagetables and placed under direct-io. This covers hugetlb pages and, conceivably, soundcard DMA buffers which were allcoated with a higher-order allocation but which weren't marked PageReserved. The patch arranges for the hugetlb implications to allocate their pages with compound page metadata, and all other higher-order allocations go back to the old way. (Andrea supplied the GFP_LEVEL_MASK fix)
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