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Date:   Thu, 17 Jan 2019 18:17:28 +0100
From:   Vlastimil Babka <>
To:     Mel Gorman <>,
        Linux-MM <>
Cc:     David Rientjes <>,
        Andrea Arcangeli <>,,, Andrew Morton <>,
        Linux List Kernel Mailing <>
Subject: Re: [PATCH 17/25] mm, compaction: Keep cached migration PFNs synced
 for unusable pageblocks

On 1/4/19 1:50 PM, Mel Gorman wrote:
> Migrate has separate cached PFNs for ASYNC and SYNC* migration on the
> basis that some migrations will fail in ASYNC mode. However, if the cached
> PFNs match at the start of scanning and pageblocks are skipped due to
> having no isolation candidates, then the sync state does not matter.
> This patch keeps matching cached PFNs in sync until a pageblock with
> isolation candidates is found.
> The actual benefit is marginal given that the sync scanner following the
> async scanner will often skip a number of pageblocks but it's useless
> work. Any benefit depends heavily on whether the scanners restarted
> recently so overall the reduction in scan rates is a mere 2.8% which
> is borderline noise.
> Signed-off-by: Mel Gorman <>

Acked-by: Vlastimil Babka <>

My easlier suggestion to check more thoroughly if pages can be migrated (which
depends on the mode) before isolating them wouldn't play nice with this :)

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