Messages in this thread | | | Subject | Re: [PATCH] mm, page_alloc: Fix a division by zero error when boosting watermarks | From | Vlastimil Babka <> | Date | Wed, 13 Feb 2019 14:42:36 +0100 |
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On 2/13/19 2:19 PM, Mel Gorman wrote: > 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 > either very early in boot or if the high watermark is very small. This > patch checks for the conditions and avoids boosting in those cases.
Hmm is it really a division by zero? The following line sets max_boost to pageblock_nr_pages if it's zero. And where would the division happen anyway?
So I wonder what's going on, your patch should AFAICS only take effect when zone->_watermark[WMARK_HIGH] is 0 or 1 to begin with, otherwise max_boost is at least 2?
Also upon closer look, I think that (prior to the patch), boost_watermark() could be reduced (thanks to the max+min capping) to
zone->watermark_boost = pageblock_nr_pages
?
> > Fixes: 1c30844d2dfe ("mm: reclaim small amounts of memory when an external fragmentation event occurs") > Reported-and-tested-by: Yury Norov <yury.norov@gmail.com> > Signed-off-by: Mel Gorman <mgorman@techsingularity.net> > --- > mm/page_alloc.c | 5 +++++ > 1 file changed, 5 insertions(+) > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index d295c9bc01a8..ae7e4ba5b9f5 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -2170,6 +2170,11 @@ static inline void boost_watermark(struct zone *zone) > > max_boost = mult_frac(zone->_watermark[WMARK_HIGH], > watermark_boost_factor, 10000); > + > + /* high watermark be be uninitialised or very small */ > + 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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