Re: [PATCH 1/6] mm/hugetlb: Fix uffd-wp during fork()

[David Hildenbrand]

On 14.04.23 01:11, Peter Xu wrote:
> There're a bunch of things that were wrong:
>
>    - Reading uffd-wp bit from a swap entry should use pte_swp_uffd_wp()
>      rather than huge_pte_uffd_wp().
>
>    - When copying over a pte, we should drop uffd-wp bit when
>      !EVENT_FORK (aka, when !userfaultfd_wp(dst_vma)).
>
>    - When doing early CoW for private hugetlb (e.g. when the parent page was
>      pinned), uffd-wp bit should be properly carried over if necessary.
>
> No bug reported probably because most people do not even care about these
> corner cases, but they are still bugs and can be exposed by the recent unit
> tests introduced, so fix all of them in one shot.
>
> Cc: linux-stable <stable@xxxxxxxxxxxxxxx>
> Fixes: bc70fbf269fd ("mm/hugetlb: handle uffd-wp during fork()")
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> ---
>   mm/hugetlb.c | 26 ++++++++++++++++----------
>   1 file changed, 16 insertions(+), 10 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index f16b25b1a6b9..7320e64aacc6 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4953,11 +4953,15 @@ static bool is_hugetlb_entry_hwpoisoned(pte_t pte)
>   
>   static void
>   hugetlb_install_folio(struct vm_area_struct *vma, pte_t *ptep, unsigned long addr,
> -		     struct folio *new_folio)
> +		      struct folio *new_folio, pte_t old)
>   {

Nit: The function now expects old to be !swap_pte. Which works perfectly 
fine with existing code -- the function name is a bit generic and 
misleading, unfortunately. IMHO, instead of factoring that functionality 
out to desperately try keeping copy_hugetlb_page_range() somewhat 
readable, we should just have factored out the complete copy+replace 
into a copy_hugetlb_page() function -- similar to the ordinary page 
handling -- which would have made copy_hugetlb_page_range() more 
readable eventually.

Anyhow, unrelated.

> +	pte_t newpte = make_huge_pte(vma, &new_folio->page, 1);
> +
>   	__folio_mark_uptodate(new_folio);
>   	hugepage_add_new_anon_rmap(new_folio, vma, addr);
> -	set_huge_pte_at(vma->vm_mm, addr, ptep, make_huge_pte(vma, &new_folio->page, 1));
> +	if (userfaultfd_wp(vma) && huge_pte_uffd_wp(old))
> +		newpte = huge_pte_mkuffd_wp(newpte);
> +	set_huge_pte_at(vma->vm_mm, addr, ptep, newpte);
>   	hugetlb_count_add(pages_per_huge_page(hstate_vma(vma)), vma->vm_mm);
>   	folio_set_hugetlb_migratable(new_folio);
>   }
> @@ -5032,14 +5036,11 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
>   			 */
>   			;
>   		} else if (unlikely(is_hugetlb_entry_hwpoisoned(entry))) {
> -			bool uffd_wp = huge_pte_uffd_wp(entry);
> -
> -			if (!userfaultfd_wp(dst_vma) && uffd_wp)
> +			if (!userfaultfd_wp(dst_vma))
>   				entry = huge_pte_clear_uffd_wp(entry);
>   			set_huge_pte_at(dst, addr, dst_pte, entry);
>   		} else if (unlikely(is_hugetlb_entry_migration(entry))) {
>   			swp_entry_t swp_entry = pte_to_swp_entry(entry);
> -			bool uffd_wp = huge_pte_uffd_wp(entry);
>   
>   			if (!is_readable_migration_entry(swp_entry) && cow) {
>   				/*
> @@ -5049,11 +5050,12 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
>   				swp_entry = make_readable_migration_entry(
>   							swp_offset(swp_entry));
>   				entry = swp_entry_to_pte(swp_entry);
> -				if (userfaultfd_wp(src_vma) && uffd_wp)
> -					entry = huge_pte_mkuffd_wp(entry);
> +				if (userfaultfd_wp(src_vma) &&
> +				    pte_swp_uffd_wp(entry))
> +					entry = pte_swp_mkuffd_wp(entry);
>   				set_huge_pte_at(src, addr, src_pte, entry);
>   			}
> -			if (!userfaultfd_wp(dst_vma) && uffd_wp)
> +			if (!userfaultfd_wp(dst_vma))
>   				entry = huge_pte_clear_uffd_wp(entry);
>   			set_huge_pte_at(dst, addr, dst_pte, entry);
>   		} else if (unlikely(is_pte_marker(entry))) {
> @@ -5114,7 +5116,8 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
>   					/* huge_ptep of dst_pte won't change as in child */
>   					goto again;
>   				}
> -				hugetlb_install_folio(dst_vma, dst_pte, addr, new_folio);
> +				hugetlb_install_folio(dst_vma, dst_pte, addr,
> +						      new_folio, src_pte_old);
>   				spin_unlock(src_ptl);
>   				spin_unlock(dst_ptl);
>   				continue;
> @@ -5132,6 +5135,9 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
>   				entry = huge_pte_wrprotect(entry);
>   			}
>   
> +			if (!userfaultfd_wp(dst_vma))
> +				entry = huge_pte_clear_uffd_wp(entry);
> +
>   			set_huge_pte_at(dst, addr, dst_pte, entry);
>   			hugetlb_count_add(npages, dst);
>   		}

LGTM

Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>

--
Thanks,

David / dhildenb