From: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Subject: 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.
Link: http://lkml.kernel.org/r/20190213143012.GT9565@xxxxxxxxxxxxxxxxxxx
Fixes: 1c30844d2dfe ("mm: reclaim small amounts of memory when an external fragmentation event occurs")
Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Reported-by: Yury Norov <yury.norov@xxxxxxxxx>
Tested-by: Yury Norov <yury.norov@xxxxxxxxx>
Tested-by: Will Deacon <will.deacon@xxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
--- a/mm/page_alloc.c~mm-page_alloc-fix-a-division-by-zero-error-when-boosting-watermarks-v2
+++ a/mm/page_alloc.c
@@ -2170,6 +2170,18 @@ static inline void boost_watermark(struc
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,
_
