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Date:   Wed, 13 Feb 2019 13:19:23 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Yury Norov <yury.norov@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-mm@...ck.org
Subject: [PATCH] mm, page_alloc: Fix a division by zero error when boosting
 watermarks

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
either very early in boot or if the high watermark is very small. This
patch checks for the conditions and avoids boosting in those cases.

Fixes: 1c30844d2dfe ("mm: reclaim small amounts of memory when an external fragmentation event occurs")
Reported-and-tested-by: Yury Norov <yury.norov@...il.com>
Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
---
 mm/page_alloc.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index d295c9bc01a8..ae7e4ba5b9f5 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2170,6 +2170,11 @@ static inline void boost_watermark(struct zone *zone)
 
 	max_boost = mult_frac(zone->_watermark[WMARK_HIGH],
 			watermark_boost_factor, 10000);
+
+	/* high watermark be be uninitialised or very small */
+	if (!max_boost)
+		return;
+
 	max_boost = max(pageblock_nr_pages, max_boost);
 
 	zone->watermark_boost = min(zone->watermark_boost + pageblock_nr_pages,

-- 
Mel Gorman
SUSE Labs

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