mm, page_alloc: fix a division by zero error when boosting watermarks v2

gormanm · torvalds · commit 94b3334cbebe · 2019-02-21T09:01:00.000-08:00
Yury Norov reported that an arm64 KVM instance could not boot since after v5.0-rc1 and could addressed by reverting the patches 1c30844 ("mm: reclaim small amounts of memory when an external 73444bc ("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. Link: http://lkml.kernel.org/r/20190213143012.GT9565@techsingularity.net Fixes: 1c30844 ("mm: reclaim small amounts of memory when an external fragmentation event occurs") Signed-off-by: Mel Gorman <mgorman@techsingularity.net> Reported-by: Yury Norov <yury.norov@gmail.com> Tested-by: Yury Norov <yury.norov@gmail.com> Tested-by: Will Deacon <will.deacon@arm.com> Acked-by: Vlastimil Babka <vbabka@suse.cz> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: David Rientjes <rientjes@google.com> Cc: Michal Hocko <mhocko@kernel.org> Cc: Catalin Marinas <catalin.marinas@arm.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git 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,
