[PATCH v2 1/2] mm/memfd: reserve hugetlb folios before allocation

[Andrew Morton]

On Tue, 14 Jan 2025 00:08:00 -0800 Vivek Kasireddy <vivek.kasireddy at intel.com> wrote:

> There are cases when we try to pin a folio but discover that it has
> not been faulted-in. So, we try to allocate it in memfd_alloc_folio()
> but there is a chance that we might encounter a crash/failure
> (VM_BUG_ON(!h->resv_huge_pages)) if there are no active reservations
> at that instant. This issue was reported by syzbot:
> 
> kernel BUG at mm/hugetlb.c:2403!
>
> ...
> 
> Therefore, to avoid this situation and fix this issue, we just need
> to make a reservation (by calling hugetlb_reserve_pages()) before
> we try to allocate the folio. This will ensure that we are properly
> doing region/subpool accounting associated with our allocation.
> 
> While at it, move subpool_inode() into hugetlb header and also
> replace the VM_BUG_ON() with WARN_ON_ONCE() as there is no need to
> crash the system in this scenario and instead we could just warn
> and fail the allocation.
> 
> ...
>
> @@ -2397,12 +2392,15 @@ struct folio *alloc_hugetlb_folio_reserve(struct hstate *h, int preferred_nid,
>  	struct folio *folio;
>  
>  	spin_lock_irq(&hugetlb_lock);
> +	if (WARN_ON_ONCE(!h->resv_huge_pages)) {
> +		spin_unlock_irq(&hugetlb_lock);
> +		return NULL;
> +	}
> +

What is is that we're warning of here?  Is there any action which
either kernel developers or the user can take to prevent this warning
from being issued?

IOW, maybe the WARN shouldn't be present?