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

Minchan Kim
2016-07-14T01:20:48Z

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.