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Commit 94b3334c authored by Mel Gorman's avatar Mel Gorman Committed by Linus Torvalds
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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: default avatarMel Gorman <mgorman@techsingularity.net>
Reported-by: default avatarYury Norov <yury.norov@gmail.com>
Tested-by: default avatarYury Norov <yury.norov@gmail.com>
Tested-by: default avatarWill Deacon <will.deacon@arm.com>
Acked-by: default avatarVlastimil 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: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus 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,