Re: hugetlb: reservation race leading to under provisioning

[Michal Hocko <mhocko@kernel.org>]

On Sun 08-01-17 11:08:30, Mike Kravetz wrote:
[...]
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 3edb759..221abdc 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1783,12 +1783,9 @@ static void return_unused_surplus_pages(struct hstate *h,
>  {
>  	unsigned long nr_pages;
>  
> -	/* Uncommit the reservation */
> -	h->resv_huge_pages -= unused_resv_pages;
> -
>  	/* Cannot return gigantic pages currently */
>  	if (hstate_is_gigantic(h))
> -		return;
> +		goto out;
>  
>  	nr_pages = min(unused_resv_pages, h->surplus_huge_pages);
>  
> @@ -1801,10 +1798,16 @@ static void return_unused_surplus_pages(struct hstate *h,
>  	 * on-line nodes with memory and will handle the hstate accounting.
>  	 */
>  	while (nr_pages--) {
> +		h->resv_huge_pages--;
> +		unused_resv_pages--;
>  		if (!free_pool_huge_page(h, &node_states[N_MEMORY], 1))
> -			break;
> +			goto out;
>  		cond_resched_lock(&hugetlb_lock);
>  	}
> +
> +out:
> +	/* Fully uncommit the reservation */
> +	h->resv_huge_pages -= unused_resv_pages;
>  }

OK, this would handle the case I was wondering about. It really deserves
a big fat comment explaining when this can happen.

Other than that this looks OK and safe enough for the stable after
second look into the code.

Thanks!
-- 
Michal Hocko
SUSE Labs

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