Re: [PATCH 1/3] mm/hugetlb: Pre-allocate pgtable pages for uffd wr-protects

[David Hildenbrand]

On 04.01.23 23:52, Peter Xu wrote:
> Userfaultfd-wp uses pte markers to mark wr-protected pages for both shmem
> and hugetlb.  Shmem has pre-allocation ready for markers, but hugetlb path
> was overlooked.
>
> Doing so by calling huge_pte_alloc() if the initial pgtable walk fails to
> find the huge ptep.  It's possible that huge_pte_alloc() can fail with high
> memory pressure, in that case stop the loop immediately and fail silently.
> This is not the most ideal solution but it matches with what we do with
> shmem meanwhile it avoids the splat in dmesg.
>
> Cc: linux-stable <stable@xxxxxxxxxxxxxxx> # 5.19+
> Fixes: 60dfaad65aa9 ("mm/hugetlb: allow uffd wr-protect none ptes")
> Reported-by: James Houghton <jthoughton@xxxxxxxxxx>
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> ---
>   mm/hugetlb.c | 13 +++++++++++--
>   1 file changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index bf7a1f628357..017d9159cddf 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6649,8 +6649,17 @@ unsigned long hugetlb_change_protection(struct vm_area_struct *vma,
>   		spinlock_t *ptl;
>   		ptep = hugetlb_walk(vma, address, psize);

if (!ptep && likely(!uffd_wp)) {
	/* Nothing to protect. */
	address |= last_addr_mask;
	continue;
} else if (!ptep) {
	...
}

Might look slightly more readable would minimize changes. This should 
work, so

Acked-by: David Hildenbrand <david@xxxxxxxxxx>

>   		if (!ptep) {
> -			address |= last_addr_mask;
> -			continue;
> +			if (!uffd_wp) {
> +				address |= last_addr_mask;
> +				continue;
> +			}
> +			/*
> +			 * Userfaultfd wr-protect requires pgtable
> +			 * pre-allocations to install pte markers.
> +			 */
> +			ptep = huge_pte_alloc(mm, vma, address, psize);
> +			if (!ptep)
> +				break;
>   		}
>   		ptl = huge_pte_lock(h, mm, ptep);
>   		if (huge_pmd_unshare(mm, vma, address, ptep)) {

--
Thanks,

David / dhildenb