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

[Peter Xu]

On Mon, Mar 03, 2025 at 10:41:05AM +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

Looks reasonable to me.  However I'm not sure whether this may cause
regression on concurrent allocations of surplus pages.

Would it be possible to stick with hugetlb_lock?  IIUC only the allocation
part of alloc_fresh_hugetlb_folio() needs the lock to be released, then
we could also update the two counters together with hugetlb_lock by open
code alloc_fresh_hugetlb_folio(), and move __prep_account_new_huge_page()
out to be after lock taken.

> 
> Finally, nr_huge_pages is less than surplus_huge_pages.
> 
> Fixes: 0c397daea1d4 ("mm, hugetlb: further simplify hugetlb allocation API")
> Signed-off-by: Liu Shixin <liushixin2@xxxxxxxxxx>
> ---
>  mm/hugetlb.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 9faa1034704ff..a900562ea7679 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2248,14 +2248,17 @@ static struct folio *alloc_surplus_hugetlb_folio(struct hstate *h,
>  	if (hstate_is_gigantic(h))
>  		return NULL;
>  
> +	mutex_lock(&h->resize_lock);
>  	spin_lock_irq(&hugetlb_lock);
>  	if (h->surplus_huge_pages >= h->nr_overcommit_huge_pages)
>  		goto out_unlock;
>  	spin_unlock_irq(&hugetlb_lock);
>  
>  	folio = alloc_fresh_hugetlb_folio(h, gfp_mask, nid, nmask);
> -	if (!folio)
> +	if (!folio) {
> +		mutex_unlock(&h->resize_lock);
>  		return NULL;
> +	}
>  
>  	spin_lock_irq(&hugetlb_lock);
>  	/*
> @@ -2268,6 +2271,7 @@ static struct folio *alloc_surplus_hugetlb_folio(struct hstate *h,
>  	if (h->surplus_huge_pages >= h->nr_overcommit_huge_pages) {
>  		folio_set_hugetlb_temporary(folio);
>  		spin_unlock_irq(&hugetlb_lock);
> +		mutex_unlock(&h->resize_lock);
>  		free_huge_folio(folio);
>  		return NULL;
>  	}
> @@ -2277,6 +2281,7 @@ static struct folio *alloc_surplus_hugetlb_folio(struct hstate *h,
>  
>  out_unlock:
>  	spin_unlock_irq(&hugetlb_lock);
> +	mutex_unlock(&h->resize_lock);
>  
>  	return folio;
>  }
> -- 
> 2.34.1
> 
> 

-- 
Peter Xu