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From:   Nadav Amit <namit@vmware.com>
To:     David Hildenbrand <david@redhat.com>
CC:     Linux MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
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        Andy Lutomirski <luto@kernel.org>,
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        Thomas Gleixner <tglx@linutronix.de>,
        Will Deacon <will@kernel.org>, Yu Zhao <yuzhao@google.com>,
        Nick Piggin <npiggin@gmail.com>
Subject: Re: [RFC PATCH 01/14] userfaultfd: set dirty and young on
 writeprotect
Thread-Topic: [RFC PATCH 01/14] userfaultfd: set dirty and young on
 writeprotect
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Date:   Wed, 20 Jul 2022 18:09:26 +0000
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On Jul 20, 2022, at 11:00 AM, David Hildenbrand <david@redhat.com> wrote:

> My patch review skills have seen better days. I thought you'd be
> removing the pte_write() check ... :( Tired eyes ...
>=20
>> Having said that, I do notice now that pte_mkdirty() should not be done
>> only this condition is fulfilled. Instead we should just have
>> something like:
>>=20
>> if (will_need) {
>> ptent =3D pte_mkyoung(ptent);
>> if (pte_write(ptent))
>> ptent =3D pte_mkdirty(ptent);
>> }
>=20
> As can_change_pte_writable() will fail if it stumbles over a !pte_dirty
> page in current code ... so I assume you would have that code before the
> actual pte_mkwrite() logic, correct?

No, I thought this should go after for 2 reasons:

1. You want to allow the PTE to be made writable following the
can_change_pte_writable().

2. You do not want to set a non-writable PTE as dirty, especially since it
might then be used to determine that the PTE can become writable. Doing so
would circumvent cases in which set_page_dirty() needs to be called and
break things down.