From 8166354c5896fb1b1b1db2497b6d62705cb04e25 Mon Sep 17 00:00:00 2001 From: Mel Gorman Date: Wed, 20 Feb 2019 22:19:49 -0800 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 a7d8cd1d83b0cc85661 ("mm: reclaim small amounts of memory when an external 7ce7a0816a06e2dcca0 ("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: a7d8cd1d83b0 ("mm: reclaim small amounts of memory when an external fragmentation event occurs") Signed-off-by: Mel Gorman Reported-by: Yury Norov Tested-by: Yury Norov Tested-by: Will Deacon Acked-by: Vlastimil Babka Cc: Andrea Arcangeli Cc: David Rientjes Cc: Michal Hocko Cc: Catalin Marinas Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/page_alloc.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 7f79b78bc8294..0b9f577b1a2ae 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, -- 2.39.5