Re: [PATCH 2/2] mm/rmap: Fixup copying of soft dirty and uffd ptes

[Peter Xu]

On Mon, Aug 24, 2020 at 06:31:28PM +1000, Alistair Popple wrote:
> During memory migration a pte is temporarily replaced with a migration
> swap pte. Some pte bits from the existing mapping such as the soft-dirty
> and uffd write-protect bits are preserved by copying these to the
> temporary migration swap pte.
> 
> However these bits are not stored at the same location for swap and
> non-swap ptes. Therefore testing these bits requires using the
> appropriate helper function for the given pte type.
> 
> Unfortunately several code locations were found where the wrong helper
> function is being used to test soft_dirty and uffd_wp bits which leads
> to them getting incorrectly set or cleared during page-migration.
> 
> Fix these by using the correct tests based on pte type.
> 
> Fixes: a5430dda8a3a ("mm/migrate: support un-addressable ZONE_DEVICE page in migration")
> Fixes: 8c3328f1f36a ("mm/migrate: migrate_vma() unmap page from vma while collecting pages")
> Fixes: f45ec5ff16a7 ("userfaultfd: wp: support swap and page migration")
> Signed-off-by: Alistair Popple <alistair@xxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
>  mm/migrate.c | 6 ++++--
>  mm/rmap.c    | 9 +++++++--
>  2 files changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index ddb64253fe3e..5bea19c496af 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -2427,9 +2427,11 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>  			entry = make_migration_entry(page, mpfn &
>  						     MIGRATE_PFN_WRITE);
>  			swp_pte = swp_entry_to_pte(entry);
> -			if (pte_soft_dirty(pte))
> +			if ((is_swap_pte(pte) && pte_swp_soft_dirty(pte))
> +				|| (!is_swap_pte(pte) && pte_soft_dirty(pte)))
>  				swp_pte = pte_swp_mksoft_dirty(swp_pte);
> -			if (pte_uffd_wp(pte))
> +			if ((is_swap_pte(pte) && pte_swp_uffd_wp(pte))
> +				|| (!is_swap_pte(pte) && pte_uffd_wp(pte)))
>  				swp_pte = pte_swp_mkuffd_wp(swp_pte);
>  			set_pte_at(mm, addr, ptep, swp_pte);

The worst case is we'll call is_swap_pte() four times for each entry. Also
considering we know it's not a pte_none() when reach here, how about:

  if (pte_present(pte)) {
    // pte handling of both soft dirty and uffd-wp
  } else {
    // swap handling of both soft dirty and uffd-wp
  }

?

Thanks,

-- 
Peter Xu