When setting the low and high watermarks we use min_wmark_pages(zone).
I guess this is to reduce the line length. But we forgot that this macro
includes zone->watermark_boost. We need to reset zone->watermark_boost
first. Otherwise the watermarks will be set inconsistently.
E.g. this could cause inconsistent values if the watermarks have been
boosted, and then you change a sysctl which triggers
__setup_per_zone_wmarks().
I strongly suspect this explains why I have seen slightly high watermarks.
Suspicious-looking zoneinfo below - notice high-low != low-min.
Node 0, zone Normal
pages free 74597
min 9582
low 34505
high 36900
https://unix.stackexchange.com/questions/525674/my-low-and-high-watermarks-seem-higher-than-predicted-by-documentation-sysctl-vm/525687
Signed-off-by: Alan Jenkins <alan.christopher.jenkins@xxxxxxxxx>
Fixes: 1c30844d2dfe ("mm: reclaim small amounts of memory when an external
fragmentation event occurs")
Cc: stable@xxxxxxxxxxxxxxx
---
Tested by compiler :-).
Ideally the commit message would be clear about what happens the
*first* time __setup_per_zone_watermarks() is called. I guess that
zone->watermark_boost is *usually* zero, or we would have noticed
some wild problems :-). However I am not familiar with how the zone
structures are allocated & initialized. Maybe there is a case where
zone->watermark_boost could contain an arbitrary unitialized value
at this point. Can we rule that out?
mm/page_alloc.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c02cff1ed56e..db9758cda6f8 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -7606,9 +7606,9 @@ static void __setup_per_zone_wmarks(void)
mult_frac(zone_managed_pages(zone),
watermark_scale_factor, 10000));
+ zone->watermark_boost = 0;
zone->_watermark[WMARK_LOW] = min_wmark_pages(zone) + tmp;
zone->_watermark[WMARK_HIGH] = min_wmark_pages(zone) + tmp * 2;
- zone->watermark_boost = 0;
spin_unlock_irqrestore(&zone->lock, flags);
}
--
2.20.1
