mm, page_alloc: Fix a division by zero error when boosting watermarks v2
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Message ID 20190213143012.GT9565@techsingularity.net
State New
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Series
  • mm, page_alloc: Fix a division by zero error when boosting watermarks v2
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Commit Message

Mel Gorman Feb. 13, 2019, 2:30 p.m. UTC
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.

Fixes: 1c30844d2dfe ("mm: reclaim small amounts of memory when an external fragmentation event occurs")
Reported-and-tested-by: Yury Norov <yury.norov@gmail.com>
Tested-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
 mm/page_alloc.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

Comments

Vlastimil Babka Feb. 13, 2019, 2:31 p.m. UTC | #1
On 2/13/19 3:30 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 very early in boot if the system has very little memory. This
> patch avoids the division by zero error.
> 
> Fixes: 1c30844d2dfe ("mm: reclaim small amounts of memory when an external fragmentation event occurs")
> Reported-and-tested-by: Yury Norov <yury.norov@gmail.com>
> Tested-by: Will Deacon <will.deacon@arm.com>
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

Thanks, sorry for the noise before.

> ---
>  mm/page_alloc.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index d295c9bc01a8..bb1c7d843ebf 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,
>

Patch
diff mbox series

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index d295c9bc01a8..bb1c7d843ebf 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,