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Re: [PATCH 1/4] mm, vmscan: Have kswapd reclaim from all zones if reclaiming and buffer_heads_over_limit -fix

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On Wed, Jul 13, 2016 at 11:00:01AM +0100, Mel Gorman wrote:
> Johannes reported that the comment about buffer_heads_over_limit in
> balance_pgdat only made sense in the context of the patch. This patch
> clarifies the reasoning and how it applies to 32 and 64 bit systems.
> 
> This is a fix to the mmotm patch
> mm-vmscan-have-kswapd-reclaim-from-all-zones-if-reclaiming-and-buffer_heads_over_limit.patch
> 
> Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> ---
>  mm/vmscan.c | 13 +++++++------
>  1 file changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index d079210d46ee..21eae17ee730 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3131,12 +3131,13 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
>  
>  		/*
>  		 * If the number of buffer_heads exceeds the maximum allowed
> -		 * then consider reclaiming from all zones. This is not
> -		 * specific to highmem which may not exist but it is it is
> -		 * expected that buffer_heads are stripped in writeback.
> -		 * Reclaim may still not go ahead if all eligible zones
> -		 * for the original allocation request are balanced to
> -		 * avoid excessive reclaim from kswapd.
> +		 * then consider reclaiming from all zones. This has a dual
> +		 * purpose -- on 64-bit systems it is expected that
> +		 * buffer_heads are stripped during active rotation. On 32-bit
> +		 * systems, highmem pages can pin lowmem memory and shrinking
> +		 * buffers can relieve lowmem pressure. Reclaim may still not

It's good but I hope we can make it more clear.

On 32-bit systems, highmem pages can pin lowmem pages storing buffer_heads
so shrinking highmem pages can relieve lowmem pressure.

If you don't think it's much readable compared to yours, feel free to drop.

Thanks.

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[PATCH 0/4] Follow-up fixes to node-lru series v1 Mel Gorman <mgorman@techsingularity.net> - 2016-07-13 12:10 +0200
  [PATCH 2/4] mm: vmstat: account per-zone stalls and pages skipped during reclaim -fix Mel Gorman <mgorman@techsingularity.net> - 2016-07-13 12:10 +0200
    Re: [PATCH 2/4] mm: vmstat: account per-zone stalls and pages  skipped during reclaim -fix Johannes Weiner <hannes@cmpxchg.org> - 2016-07-13 15:20 +0200
    Re: [PATCH 2/4] mm: vmstat: account per-zone stalls and pages skipped  during reclaim -fix Vlastimil Babka <vbabka@suse.cz> - 2016-07-14 16:50 +0200
  [PATCH 1/4] mm, vmscan: Have kswapd reclaim from all zones if reclaiming and buffer_heads_over_limit -fix Mel Gorman <mgorman@techsingularity.net> - 2016-07-13 12:10 +0200
    Re: [PATCH 1/4] mm, vmscan: Have kswapd reclaim from all zones if  reclaiming and buffer_heads_over_limit -fix Johannes Weiner <hannes@cmpxchg.org> - 2016-07-13 15:20 +0200
    Re: [PATCH 1/4] mm, vmscan: Have kswapd reclaim from all zones if  reclaiming and buffer_heads_over_limit -fix Minchan Kim <minchan@kernel.org> - 2016-07-14 03:30 +0200
  [PATCH 3/4] mm, page_alloc: fix dirtyable highmem calculation Mel Gorman <mgorman@techsingularity.net> - 2016-07-13 12:10 +0200
    Re: [PATCH 3/4] mm, page_alloc: fix dirtyable highmem calculation Johannes Weiner <hannes@cmpxchg.org> - 2016-07-13 15:20 +0200
    Re: [PATCH 3/4] mm, page_alloc: fix dirtyable highmem calculation Vlastimil Babka <vbabka@suse.cz> - 2016-07-14 17:30 +0200
  [PATCH 4/4] mm: move most file-based accounting to the node -fix Mel Gorman <mgorman@techsingularity.net> - 2016-07-13 12:10 +0200
    Re: [PATCH 4/4] mm: move most file-based accounting to the node -fix Johannes Weiner <hannes@cmpxchg.org> - 2016-07-13 15:20 +0200

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