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-s...
Signed-off-by: Alan Jenkins alan.christopher.jenkins@gmail.com Fixes: 1c30844d2dfe ("mm: reclaim small amounts of memory when an external fragmentation event occurs") Cc: stable@vger.kernel.org ---
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); }