Re: [PATCH] mm/hugetlb: add missing hugetlb_lock in __unmap_hugepage_range()

[Sidhartha Kumar <sidhartha.kumar@oracle.com>]

On 8/22/25 1:58 AM, Jeongjun Park wrote:
> When restoring a reservation for an anonymous page, we need to check to
> freeing a surplus. However, __unmap_hugepage_range() causes data race
> because it reads h->surplus_huge_pages without the protection of
> hugetlb_lock.
> 
> Therefore, we need to add missing hugetlb_lock.
> 

Makes sense as alloc_surplus_hugetlb_folio() takes the hugetlb_lock when 
reading the hstate.

Reviewed-by: Sidhartha Kumar <sidhartha.kumar@oracle.com>

> Reported-by: syzbot+417aeb05fd190f3a6da9@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=417aeb05fd190f3a6da9
> Fixes: df7a6d1f6405 ("mm/hugetlb: restore the reservation if needed")
> Signed-off-by: Jeongjun Park <aha310510@gmail.com>
> ---
>   mm/hugetlb.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 753f99b4c718..e8d95a314df2 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -5951,6 +5951,8 @@ void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct *vma,
>   		 * If there we are freeing a surplus, do not set the restore
>   		 * reservation bit.
>   		 */
> +		spin_lock_irq(&hugetlb_lock);
> +
>   		if (!h->surplus_huge_pages && __vma_private_lock(vma) &&
>   		    folio_test_anon(folio)) {
>   			folio_set_hugetlb_restore_reserve(folio);
> @@ -5958,6 +5960,7 @@ void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct *vma,
>   			adjust_reservation = true;
>   		}
>   
> +		spin_unlock_irq(&hugetlb_lock);
>   		spin_unlock(ptl);
>   
>   		/*
> --
>