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@techsingularity.net
Fixes: 1c30844d2dfe ("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>

1 files changed, 12 insertions, 0 deletions

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 7f79b78bc829..0b9f577b1a2a 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,