Commit 94b3334c authored Feb 20, 2019 by Mel Gorman Committed by Linus Torvalds Feb 21, 2019

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

  1c30844d ("mm: reclaim small amounts of memory when an external
  73444bc4 ("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: 1c30844d ("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>

parent 311ade0e

mm/page_alloc.c

+12 −0

Original line number	Diff line number	Diff line
		@@ -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,

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