Re: [PATCH v2] mm: Don't fault around userfaultfd-registered regions on reads

[Andrea Arcangeli]

On Thu, Dec 03, 2020 at 11:10:18PM -0500, Andrea Arcangeli wrote:
> from the pte, one that cannot ever be set in any swp entry today. I
> assume it can't be _PAGE_SWP_UFFD_WP since that already can be set but
> you may want to verify it...

I thought more about the above, and I think the already existing
pte_swp_mkuffd_wp will just be enough without having to reserve an
extra bitflag if we encode it as a non migration entry.

The check:

if (!pte_present && !pte_none && pte_swp_uffd_wp && not_anonymous_vma && !is_migration_entry)

should be enough to disambiguate it. When setting it, it'd be enough
to set the pte to the value _PAGE_SWP_UFFD_WP.

Although if you prefer to check for:

if (!pte_present && !pte_none && swp_type == 1 && swp_offset == 0 && not_anonymous_vma && !is_migration_entry)

that would do as well.

It's up to you, just my preference is to reuse _PAGE_SWP_UFFD_WP since
it has already to exist, there are already all the pte_swp_*uffd*
methods available or uffd-wp cannot work.

Thanks,
Andrea