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Subject: Re: [PATCH] mm: fix setting the high and low watermarks
To:     Alan Jenkins <alan.christopher.jenkins@gmail.com>,
        Mel Gorman <mgorman@techsingularity.net>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org
References: <20190621114325.711-1-alan.christopher.jenkins@gmail.com>
From:   Vlastimil Babka <vbabka@suse.cz>
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Message-ID: <3d15b808-b7cd-7379-a6a9-d3cf04b7dcec@suse.cz>
Date:   Fri, 21 Jun 2019 14:09:31 +0200
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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.jenkins@gmail.com>
> Fixes: 1c30844d2dfe ("mm: reclaim small amounts of memory when an external
>                       fragmentation event occurs")
> Cc: stable@vger.kernel.org

Nice catch, thanks!

Acked-by: Vlastimil Babka <vbabka@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);