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Subject: Re: [PATCH v2 12/39] x86/mm: Update ptep_set_wrprotect() and
 pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE_COW
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <65CF3B29-BF53-4BA3-89D9-5398CEB7F813@gmail.com>
Date:   Mon, 3 Oct 2022 16:25:27 -0700
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References: <20220929222936.14584-1-rick.p.edgecombe@intel.com>
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To:     "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>
Precedence: bulk

On Oct 3, 2022, at 4:20 PM, Nadav Amit <nadav.amit@gmail.com> wrote:

> On Oct 3, 2022, at 4:17 PM, Nadav Amit <nadav.amit@gmail.com> wrote:
>=20
>> On Oct 3, 2022, at 3:28 PM, Edgecombe, Rick P =
<rick.p.edgecombe@intel.com> wrote:
>>=20
>>> On Mon, 2022-10-03 at 11:11 -0700, Nadav Amit wrote:
>>>> Did you have a look at ptep_set_access_flags() and friends and
>>>> checked they
>>>> do not need to be changed too?=20
>>>=20
>>> ptep_set_access_flags() doesn't actually set any additional dirty =
bits
>>> on x86, so I think it's ok.
>>=20
>> Are you sure about that? (lost my confidence today so I am hesitant).
>>=20
>> Looking on insert_pfn(), I see:
>>=20
>>                       entry =3D maybe_mkwrite(pte_mkdirty(entry), =
vma);
>>                       if (ptep_set_access_flags(vma, addr, pte, =
entry, 1)) ...
>>=20
>> This appears to set the dirty bit while potentially leaving the =
write-bit
>> clear. This is the scenario you want to avoid, no?
>=20
> No. I am not paying attention. Ignore.

Sorry for the spam. Just this =E2=80=9Cdirty=E2=80=9D argument is =
confusing. This indeed
seems like a flow that can set the dirty bit. I think.