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Date:   Thu, 2 Nov 2017 13:04:33 +0000
From:   Mel Gorman <>
To:     Vlastimil Babka <>
Cc:     Andrew Morton <>,,, David Rientjes <>,
        Joonsoo Kim <>
Subject: Re: [PATCH 1/3] mm, compaction: extend pageblock_skip_persistent()
 to all compound pages

On Thu, Nov 02, 2017 at 01:17:04PM +0100, Vlastimil Babka wrote:
> The pageblock_skip_persistent() function checks for HugeTLB pages of pageblock
> order. When clearing pageblock skip bits for compaction, the bits are not
> cleared for such pageblocks, because they cannot contain base pages suitable
> for migration, nor free pages to use as migration targets.
> This optimization can be simply extended to all compound pages of order equal
> or larger than pageblock order, because migrating such pages (if they support
> it) cannot help sub-pageblock fragmentation. This includes THP's and also
> gigantic HugeTLB pages, which the current implementation doesn't persistently
> skip due to a strict pageblock_order equality check and not recognizing tail
> pages.
> While THP pages are generally less "persistent" than HugeTLB, we can still
> expect that if a THP exists at the point of __reset_isolation_suitable(), it
> will exist also during the subsequent compaction run. The time difference here
> could be actually smaller than between a compaction run that sets a
> (non-persistent) skip bit on a THP, and the next compaction run that observes
> it.
> Signed-off-by: Vlastimil Babka <>

Acked-by: Mel Gorman <>

Mel Gorman

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