Re: [PATCH] mm/hugetlb: avoid hardcoding while checking if cma is reserved

[Roman Gushchin]

On Mon, Jul 06, 2020 at 08:44:05PM +1200, Barry Song wrote:

Hello, Barry!

> hugetlb_cma[0] can be NULL due to various reasons, for example, node0 has
> no memory. Thus, NULL hugetlb_cma[0] doesn't necessarily mean cma is not
> enabled. gigantic pages might have been reserved on other nodes.

Just curious, is it a real-life problem you've seen? If so, I wonder how
you're using the hugetlb_cma option, and what's the outcome?

> 
> Fixes: cf11e85fc08c ("mm: hugetlb: optionally allocate gigantic hugepages using cma")
> Cc: Roman Gushchin <guro@xxxxxx>
> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> Cc: Jonathan Cameron <jonathan.cameron@xxxxxxxxxx>
> Signed-off-by: Barry Song <song.bao.hua@xxxxxxxxxxxxx>
> ---
>  mm/hugetlb.c | 18 +++++++++++++++---
>  1 file changed, 15 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 57ece74e3aae..603aa854aa89 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2571,9 +2571,21 @@ static void __init hugetlb_hstate_alloc_pages(struct hstate *h)
>  
>  	for (i = 0; i < h->max_huge_pages; ++i) {
>  		if (hstate_is_gigantic(h)) {
> -			if (IS_ENABLED(CONFIG_CMA) && hugetlb_cma[0]) {
> -				pr_warn_once("HugeTLB: hugetlb_cma is enabled, skip boot time allocation\n");
> -				break;
> +			if (IS_ENABLED(CONFIG_CMA)) {
> +				int nid;
> +				bool cma_reserved = false;
> +
> +				for_each_node_state(nid, N_ONLINE) {
> +					if (hugetlb_cma[nid]) {
> +						pr_warn_once("HugeTLB: hugetlb_cma is reserved,"
> +								"skip boot time allocation\n");
> +						cma_reserved = true;
> +						break;
> +					}
> +				}
> +
> +				if (cma_reserved)
> +					break;

It's a valid problem, and I like to see it fixed. But I wonder if it would be better
to introduce a new helper bool hugetlb_cma_enabled()? And move both IS_ENABLED(CONFIG_CMA)
and hugetlb_cma[nid] checks there?

Thank you!