Re: [PATCH v2] mm/hugetlb: update nr_huge_pages and surplus_huge_pages together

[Oscar Salvador]

On Tue, Mar 04, 2025 at 04:38:41PM +0800, Liu Shixin wrote:
> In alloc_surplus_hugetlb_folio(), we increase nr_huge_pages and
> surplus_huge_pages separately. In the middle window, if we set
> nr_hugepages to smaller and satisfy count < persistent_huge_pages(h),
> the surplus_huge_pages will be increased by adjust_pool_surplus().
> 
> After adding delay in the middle window, we can reproduce the problem
> easily by following step:
> 
>  1. echo 3 > /proc/sys/vm/nr_overcommit_hugepages
>  2. mmap two hugepages. When nr_huge_pages=2 and surplus_huge_pages=1,
>     goto step 3.
>  3. echo 0 > /proc/sys/vm/nr_huge_pages
> 
> Finally, nr_huge_pages is less than surplus_huge_pages.
> 
> To fix the problem, call only_alloc_fresh_hugetlb_folio() instead and
> move down __prep_account_new_huge_page() into the hugetlb_lock.
> 
> Fixes: 0c397daea1d4 ("mm, hugetlb: further simplify hugetlb allocation API")
> Signed-off-by: Liu Shixin <liushixin2@xxxxxxxxxx>
> ---
>  mm/hugetlb.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 9faa1034704ff..0b02ea1c39e63 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2253,11 +2253,19 @@ static struct folio *alloc_surplus_hugetlb_folio(struct hstate *h,
>  		goto out_unlock;
>  	spin_unlock_irq(&hugetlb_lock);
>  
> -	folio = alloc_fresh_hugetlb_folio(h, gfp_mask, nid, nmask);
> +	folio = only_alloc_fresh_hugetlb_folio(h, gfp_mask, nid, nmask, NULL);
>  	if (!folio)
>  		return NULL;
>  
> +	hugetlb_vmemmap_optimize_folio(h, folio);
> +
>  	spin_lock_irq(&hugetlb_lock);
> +	/*
> +	 * Update nr_huge_pages and surplus_huge_pages together,
> +	 * otherwise it might confuse persistent_huge_pages() momentarily.
> +	 */
> +	__prep_account_new_huge_page(h, nid);

It would be great if we could pair up this one with the actual increase
of surplus pages, but then free_huge_folio() will decrease the
nr_huge_pages if the pool changed.

Also, that comment makes me think that__prep_account_new_huge_page()
will adjust both counters, so maybe just go with something like

"nr_huge_pages needs to be adjusted within the same lock cycle
as surplus_pages, otherwise..."


-- 
Oscar Salvador
SUSE Labs