Re: [PATCH 4/6] mm, compaction: always skip compound pages by order in migrate scanner

From: Mel Gorman
Date: Fri Jun 19 2015 - 09:58:35 EST

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 <vbabka@xxxxxxx>

Acked-by: Mel Gorman <mgorman@xxxxxxx>

Mel Gorman
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