On Wed, 5 Dec 2018, Andrea Arcangeli wrote: > __GFP_COMPACT_ONLY gave an hope it could give some middle ground but > it shows awful compaction results, it basically destroys compaction > effectiveness and we know why (COMPACT_SKIPPED must call reclaim or > compaction can't succeed because there's not enough free memory in the > node). If somebody used MADV_HUGEPAGE compaction should still work and > not fail like that. Compaction would fail to be effective even in the > local node where __GFP_THISNODE didn't fail. Worst of all it'd fail > even on non-NUMA systems (that would be easy to fix though by making > the HPAGE_PMD_ORDER check conditional to NUMA being enabled at > runtime). >
Note that in addition to COMPACT_SKIPPED that you mention, compaction can fail with COMPACT_COMPLETE, meaning the full scan has finished without freeing a hugepage, or COMPACT_DEFERRED, meaning that doing another scan is unlikely to produce a different result. COMPACT_SKIPPED makes sense to do reclaim if it can become accessible to isolate_freepages() and hopefully another allocator does not allocate from these newly freed pages before compaction can scan the zone again. For COMPACT_COMPLETE and COMPACT_DEFERRED, reclaim is unlikely to ever help.
