On Fri, 21 Jun 2019, Alan Jenkins wrote:
> When setting the low and high watermarks we use min_wmark_pages(zone).
> I guess this was to reduce the line length. Then this macro was modified
> to include zone->watermark_boost. So we needed to set watermark_boost
> before we set the high and low watermarks... but we did not.
>
> It seems mostly harmless. It might set the watermarks a bit higher than
> needed: when 1) the watermarks have been "boosted" and 2) you then
> triggered __setup_per_zone_wmarks() (by setting one of the sysctls, or
> hotplugging memory...).
>
> I noticed it because it also breaks the documented equality
> (high - low == low - min). Below is an example of reproducing the bug.
>
> First sample. Equality is met (high - low == low - min):
>
> Node 0, zone Normal
> pages free 11962
> min 9531
> low 11913
> high 14295
> spanned 1173504
> present 1173504
> managed 1134235
>
> A later sample. Something has caused us to boost the watermarks:
>
> Node 0, zone Normal
> pages free 12614
> min 10043
> low 12425
> high 14807
>
> Now trigger the watermarks to be recalculated. "cd /proc/sys/vm" and
> "cat watermark_scale_factor > watermark_scale_factor". Then the watermarks
> are boosted inconsistently. The equality is broken:
>
> Node 0, zone Normal
> pages free 12412
> min 9531
> low 12425
> high 14807
>
> 14807 - 12425 = 2382
> 12425 - 9531 = 2894
>
> Co-developed-by: Vlastimil Babka <vbabka@xxxxxxx>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> Signed-off-by: Alan Jenkins <alan.christopher.jenkins@xxxxxxxxx>
> Fixes: 1c30844d2dfe ("mm: reclaim small amounts of memory when an external
> fragmentation event occurs")
> Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
