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mm, compaction: always skip all compound pages by order in migrate scanner
The compaction migrate scanner tries to skip THP pages by their order, to reduce number of iterations for pages it cannot isolate. The check is only done if PageLRU() is true, which means it applies to THP pages, but not e.g. hugetlbfs pages or any other non-LRU compound pages, which we have to iterate by base pages. This limitation comes from the assumption that it's only safe to read compound_order() when we have the zone's lru_lock and THP cannot be split under us. But the only danger (after filtering out order values that are not below MAX_ORDER, to prevent overflows) is that we skip too much or too little after reading a bogus compound_order() due to a rare race. This is the same reasoning as patch 99c0fd5e ("mm, compaction: skip buddy pages by their order in the migrate scanner") introduced for unsafely reading PageBuddy() order. After this patch, all pages are tested for PageCompound() and we skip them by compound_order(). The test is done after the test for balloon_page_movable() as we don't want to assume if balloon pages (or other pages with own isolation and migration implementation if a generic API gets implemented) are compound or not. When tested with stress-highalloc from mmtests on 4GB system with 1GB hugetlbfs pages, the vmstat compact_migrate_scanned count decreased by 15%. [kirill.shutemov@linux.intel.com: change PageTransHuge checks to PageCompound for different series was squashed here] Signed-off-by:Vlastimil Babka <vbabka@suse.cz> Cc: Minchan Kim <minchan@kernel.org> Acked-by: