[PATCH] mm: fix setting the high and low watermarks

[Alan Jenkins <alan.christopher.jenkins@gmail.com>]

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@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);
 	}
-- 
2.20.1