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SubjectRe: [PATCH] mm, page_alloc: Fix a division by zero error when boosting watermarks v2
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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,
>

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