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

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Date:   Fri, 4 Dec 2020 13:12:56 -0500
From:   Andrea Arcangeli <aarcange@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Hugh Dickins <hughd@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v2] mm: Don't fault around userfaultfd-registered regions
 on reads

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