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

["Mika Penttilä" <mpenttil@redhat.com>]

On 14.4.2023 17.09, Peter Xu wrote:
> On Fri, Apr 14, 2023 at 12:45:29PM +0300, Mika Penttilä wrote:
>>>    		} 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);
> 
> [1]
> 
>>>    			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);
> 
> [2]
> 
>>> -				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);
>>
>>
>> This looks interesting with pte_swp_uffd_wp and pte_swp_mkuffd_wp ?
> 
> Could you explain what do you mean?
> 

Yes like you noticed also you called pte_swp_mkuffd_wp(entry) iff 
pte_swp_uffd_wp(entry) which is of course a nop.

But the fixup not dropping the temp var should work.

> I think these helpers are the right ones to use, as afaict hugetlb
> migration should follow the same pte format with !hugetlb.  However, I
> noticed I did it wrong when dropping the temp var - when at [1], "entry"
> still points to the src entry, but at [2] it's already pointing to the
> newly created one..  so I think I can't drop the var, a fixup should like:
> 
> ===8<===
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 083aae35bff8..cd3a9d8f4b70 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -5041,6 +5041,7 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
>                          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 = pte_swp_uffd_wp(entry);
> 
>                          if (!is_readable_migration_entry(swp_entry) && cow) {
>                                  /*
> @@ -5050,8 +5051,7 @@ 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) &&
> -                                   pte_swp_uffd_wp(entry))
> +                               if (userfaultfd_wp(src_vma) && uffd_wp)
>                                          entry = pte_swp_mkuffd_wp(entry);
>                                  set_huge_pte_at(src, addr, src_pte, entry);
> ===8<===
> 
> Besides, did I miss something else?
> 
> Thanks,
> 

--Mika