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From: Will Deacon <will.deacon@arm.com>
To: Mel Gorman <mgorman@techsingularity.net>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Yury Norov <yury.norov@gmail.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	Andrea Arcangeli <aarcange@redhat.com>,
	David Rientjes <rientjes@google.com>,
	Michal Hocko <mhocko@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org
Subject: Re: [PATCH] mm, page_alloc: Fix a division by zero error when boosting watermarks
Date: Wed, 13 Feb 2019 13:20:57 +0000	[thread overview]
Message-ID: <20190213132056.GE5875@brain-police> (raw)
In-Reply-To: <20190213131923.GQ9565@techsingularity.net>

On Wed, Feb 13, 2019 at 01:19:23PM +0000, 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.
> 
> 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,

I can confirm that this also allows my KVM guest to boot:

Tested-by: Will Deacon <will.deacon@arm.com>

Will

WARNING: multiple messages have this Message-ID (diff)
From: Will Deacon <will.deacon@arm.com>
To: Mel Gorman <mgorman@techsingularity.net>
Cc: Andrea Arcangeli <aarcange@redhat.com>,
	Yury Norov <yury.norov@gmail.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	linux-kernel@vger.kernel.org, Michal Hocko <mhocko@kernel.org>,
	linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org,
	David Rientjes <rientjes@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH] mm, page_alloc: Fix a division by zero error when boosting watermarks
Date: Wed, 13 Feb 2019 13:20:57 +0000	[thread overview]
Message-ID: <20190213132056.GE5875@brain-police> (raw)
In-Reply-To: <20190213131923.GQ9565@techsingularity.net>

On Wed, Feb 13, 2019 at 01:19:23PM +0000, 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.
> 
> 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,

I can confirm that this also allows my KVM guest to boot:

Tested-by: Will Deacon <will.deacon@arm.com>

Will

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linux-arm-kernel@lists.infradead.org
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  reply	other threads:[~2019-02-13 13:21 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-13 13:19 [PATCH] mm, page_alloc: Fix a division by zero error when boosting watermarks Mel Gorman
2019-02-13 13:19 ` Mel Gorman
2019-02-13 13:20 ` Will Deacon [this message]
2019-02-13 13:20   ` Will Deacon
2019-02-13 13:42 ` Vlastimil Babka
2019-02-13 13:42   ` Vlastimil Babka
2019-02-13 14:15   ` Mel Gorman
2019-02-13 14:15     ` Mel Gorman
2019-02-13 14:30 ` [PATCH] mm, page_alloc: Fix a division by zero error when boosting watermarks v2 Mel Gorman
2019-02-13 14:30   ` Mel Gorman
2019-02-13 14:31   ` Vlastimil Babka
2019-02-13 14:31     ` Vlastimil Babka

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