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 <[email protected]>
> Signed-off-by: Mel Gorman <[email protected]>
> ---
> 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,
>
