Re: [PATCH] mm, hugetlb: Avoid passing a null nodemask when there is mbind policy

[Vlastimil Babka <vbabka@suse.cz>]

On 4/15/25 14:15, Oscar Salvador wrote:
> Before trying to allocate a page, gather_surplus_pages() sets up a nodemask
> for the nodes we can allocate from, but instead of passing the nodemask
> down the road to the page allocator, it iterates over the nodes within that
> nodemask right there, meaning that the page allocator will receive a preferred_nid
> and a null nodemask.
> 
> This is a problem when using a memory policy, because it might be that
> the page allocator ends up using a node as a fallback which is not
> represented in the policy.
> 
> Avoid that by passing the nodemask directly to the page allocator, so it can
> filter out fallback nodes that are not part of the nodemask.

It will also try the fallbacks using numa distance and not incrementing nid.

> Signed-off-by: Oscar Salvador <osalvador@suse.de>

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

> ---
>  mm/hugetlb.c | 22 ++++++----------------
>  1 file changed, 6 insertions(+), 16 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index ccc4f08f8481..5e1cba0f835f 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2419,7 +2419,6 @@ static int gather_surplus_pages(struct hstate *h, long delta)
>  	long i;
>  	long needed, allocated;
>  	bool alloc_ok = true;
> -	int node;
>  	nodemask_t *mbind_nodemask, alloc_nodemask;
>  
>  	mbind_nodemask = policy_mbind_nodemask(htlb_alloc_mask(h));
> @@ -2443,21 +2442,12 @@ static int gather_surplus_pages(struct hstate *h, long delta)
>  	for (i = 0; i < needed; i++) {
>  		folio = NULL;
>  
> -		/* Prioritize current node */
> -		if (node_isset(numa_mem_id(), alloc_nodemask))
> -			folio = alloc_surplus_hugetlb_folio(h, htlb_alloc_mask(h),
> -					numa_mem_id(), NULL);
> -
> -		if (!folio) {
> -			for_each_node_mask(node, alloc_nodemask) {
> -				if (node == numa_mem_id())
> -					continue;
> -				folio = alloc_surplus_hugetlb_folio(h, htlb_alloc_mask(h),
> -						node, NULL);
> -				if (folio)
> -					break;
> -			}
> -		}
> +		/*
> +		 * It is okay to use NUMA_NO_NODE because we use numa_mem_id()
> +		 * down the road to pick the current node if that is the case.
> +		 */
> +		folio = alloc_surplus_hugetlb_folio(h, htlb_alloc_mask(h),
> +						    NUMA_NO_NODE, &alloc_nodemask);
>  		if (!folio) {
>  			alloc_ok = false;
>  			break;