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Date:   Wed, 13 Feb 2019 14:30:13 +0000
From:   Mel Gorman <>
To:     Andrew Morton <>
Cc:     Yury Norov <>,
        Vlastimil Babka <>,
        Andrea Arcangeli <>,
        David Rientjes <>,
        Michal Hocko <>,
        Will Deacon <>,
        Catalin Marinas <>,,,
Subject: [PATCH] mm, page_alloc: Fix a division by zero error when boosting
 watermarks v2

Yury Norov reported that an arm64 KVM instance could not boot since after
v5.0-rc1 and could addressed by reverting the patches

1c30844d2dfe272d58c ("mm: reclaim small amounts of memory when an external
73444bc4d8f92e46a20 ("mm, page_alloc: do not wake kswapd with zone lock held")

The problem is that a division by zero error is possible if boosting
occurs very early in boot if the system has very little memory. This
patch avoids the division by zero error.

Fixes: 1c30844d2dfe ("mm: reclaim small amounts of memory when an external fragmentation event occurs")
Reported-and-tested-by: Yury Norov <>
Tested-by: Will Deacon <>
Signed-off-by: Mel Gorman <>
 mm/page_alloc.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index d295c9bc01a8..bb1c7d843ebf 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2170,6 +2170,18 @@ static inline void boost_watermark(struct zone *zone)
 	max_boost = mult_frac(zone->_watermark[WMARK_HIGH],
 			watermark_boost_factor, 10000);
+	/*
+	 * high watermark may be uninitialised if fragmentation occurs
+	 * very early in boot so do not boost. We do not fall
+	 * through and boost by pageblock_nr_pages as failing
+	 * allocations that early means that reclaim is not going
+	 * to help and it may even be impossible to reclaim the
+	 * boosted watermark resulting in a hang.
+	 */
+	if (!max_boost)
+		return;
 	max_boost = max(pageblock_nr_pages, max_boost);
 	zone->watermark_boost = min(zone->watermark_boost + pageblock_nr_pages,

Mel Gorman

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