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From: Andrea Arcangeli <aarcange@redhat.com>
To: Peter Xu <peterx@redhat.com>
Cc: Hugh Dickins <hughd@google.com>,
	Matthew Wilcox <willy@infradead.org>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Mike Rapoport <rppt@linux.vnet.ibm.com>,
	David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH v2] mm: Don't fault around userfaultfd-registered regions on reads
Date: Fri, 4 Dec 2020 13:12:56 -0500	[thread overview]
Message-ID: <X8p8KNJXD3aK9TkF@redhat.com> (raw)
In-Reply-To: <X8m2qv9h7+e79UjJ@redhat.com>

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


  parent reply	other threads:[~2020-12-04 18:15 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-30 23:06 Peter Xu
2020-12-01  9:30 ` David Hildenbrand
2020-12-01 12:59 ` Matthew Wilcox
2020-12-01 22:30   ` Peter Xu
2020-12-02  0:02     ` Andrea Arcangeli
2020-12-02 22:37       ` Hugh Dickins
2020-12-02 23:41         ` Peter Xu
2020-12-03  5:36           ` Hugh Dickins
2020-12-03 18:02             ` Peter Xu
2020-12-03 19:44               ` Andrea Arcangeli
2020-12-04  2:30                 ` Peter Xu
2020-12-04  4:10                   ` Andrea Arcangeli
2020-12-04  5:59                     ` Hugh Dickins
2020-12-04 16:50                       ` Peter Xu
2020-12-04 18:12                     ` Andrea Arcangeli [this message]
2020-12-04 19:23                       ` Peter Xu
2020-12-04 19:37                         ` Andrea Arcangeli
2020-12-04 20:21                           ` Peter Xu

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