linux-kernel - Re: [PATCH] mm, page_alloc: Fix a division by zero error when boosting watermarks

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Message-ID: <295be99c-d09a-5572-fa49-2673a62c295b@suse.cz>
Date:   Wed, 13 Feb 2019 14:42:36 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Yury Norov <yury.norov@...il.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-mm@...ck.org
Subject: Re: [PATCH] mm, page_alloc: Fix a division by zero error when
 boosting watermarks

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@...il.com>
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.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,
>