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 <vba...@suse.cz>
Signed-off-by: Vlastimil Babka <vba...@suse.cz>
Signed-off-by: Alan Jenkins <alan.christopher.jenk...@gmail.com>
Fixes: 1c30844d2dfe ("mm: reclaim small amounts of memory when an external
fragmentation event occurs")
Acked-by: Mel Gorman <mgor...@techsingularity.net>
---
Changes since v1:
Use Vlastimil's suggested code. It is much cleaner, thanks :-).
I considered this "Co-developed-by" and s-o-b credit.
Update commit message to be specific about expected effects.
Node data is always allocated with kzalloc(). So there is no risk of
the code reading arbitrary unintialized data from ->watermark_boost,
the first time it is run.
AFAICT the bug is mostly harmless. I do not require a -stable port.
I leave it to anyone else, if they think it's worth adding
"Cc: sta...@vger.kernel.org".
mm/page_alloc.c | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c02cff1ed56e..01233705e490 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -7570,6 +7570,7 @@ static void __setup_per_zone_wmarks(void)
for_each_zone(zone) {
u64 tmp;
+ unsigned long wmark_min;
spin_lock_irqsave(&zone->lock, flags);
tmp = (u64)pages_min * zone_managed_pages(zone);
@@ -7588,13 +7589,13 @@ static void __setup_per_zone_wmarks(void)
min_pages = zone_managed_pages(zone) / 1024;
min_pages = clamp(min_pages, SWAP_CLUSTER_MAX, 128UL);
- zone->_watermark[WMARK_MIN] = min_pages;
+ wmark_min = min_pages;
} else {
/*
* If it's a lowmem zone, reserve a number of pages
* proportionate to the zone's size.
*/
- zone->_watermark[WMARK_MIN] = tmp;
+ wmark_min = tmp;
}
/*
@@ -7606,8 +7607,9 @@ static void __setup_per_zone_wmarks(void)
mult_frac(zone_managed_pages(zone),
watermark_scale_factor, 10000));
- zone->_watermark[WMARK_LOW] = min_wmark_pages(zone) + tmp;
- zone->_watermark[WMARK_HIGH] = min_wmark_pages(zone) + tmp * 2;
+ zone->_watermark[WMARK_MIN] = wmark_min;
+ zone->_watermark[WMARK_LOW] = wmark_min + tmp;
+ zone->_watermark[WMARK_HIGH] = wmark_min + tmp * 2;
zone->watermark_boost = 0;
spin_unlock_irqrestore(&zone->lock, flags);
--
2.20.1
