On 6/21/19 1:43 PM, Alan Jenkins wrote:
> 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.jenk...@gmail.com>
> Fixes: 1c30844d2dfe ("mm: reclaim small amounts of memory when an external
> fragmentation event occurs")
> Cc: sta...@vger.kernel.org
Nice catch, thanks!
Acked-by: Vlastimil Babka <vba...@suse.cz>
Personally I would implement it a bit differently, see below. If you
agree, it's fine if you keep the authorship of the whole patch.
> ---
>
> 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?
Dunno if there's some arch override, but generic_alloc_nodedata() uses
kzalloc() so it's zeroed.
-----8<-----
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index d66bc8abe0af..3b2f0cedf78e 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -7624,6 +7624,7 @@ 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);
@@ -7642,13 +7643,13 @@ 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;
}
/*
@@ -7660,8 +7661,9 @@ 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);
