Messages in this thread | | | Date | Wed, 13 Feb 2019 14:15:18 +0000 | From | Mel Gorman <> | Subject | Re: [PATCH] mm, page_alloc: Fix a division by zero error when boosting watermarks |
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On Wed, Feb 13, 2019 at 02:42:36PM +0100, Vlastimil Babka wrote: > 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? >
The issue can occur if pageblock_nr_pages is also zero or not yet initialised. It means the changelog is misleading because it has to trigger very early in boot as happened with Yury.
> 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 >
I don't think it's worth being fancy about it if we're hitting fragmentation issues that early in boot.
-- Mel Gorman SUSE Labs
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