On Wed, Jun 10, 2015 at 11:32:32AM +0200, Vlastimil Babka wrote:
> The compaction migrate scanner tries to skip compound 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 99c0fd5e51c4 ("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%.
>
> Signed-off-by: Vlastimil Babka <[email protected]>
Acked-by: Mel Gorman <[email protected]>
--
Mel Gorman
SUSE Labs
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