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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: <64313344833c3b1701002a347d539e69276b66fb.camel@intel.com>
Date:   Mon, 3 Oct 2022 16:17:51 -0700
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References: <20220929222936.14584-1-rick.p.edgecombe@intel.com>
 <20220929222936.14584-13-rick.p.edgecombe@intel.com>
 <E5D7151E-B5A6-4BEA-9642-ECCFC28F8C8E@gmail.com>
 <64313344833c3b1701002a347d539e69276b66fb.camel@intel.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>
Precedence: bulk

On Oct 3, 2022, at 3:28 PM, Edgecombe, Rick P =
<rick.p.edgecombe@intel.com> wrote:

> 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.

Are you sure about that? (lost my confidence today so I am hesitant).

Looking on insert_pfn(), I see:

                        entry =3D maybe_mkwrite(pte_mkdirty(entry), =
vma);
                        if (ptep_set_access_flags(vma, addr, pte, entry, =
1)) ...

This appears to set the dirty bit while potentially leaving the =
write-bit
clear. This is the scenario you want to avoid, no?

>> Perhaps you should at least add some
>> assertion just to ensure nothing breaks.
>=20
> You mean in ptep_set_access_flags()? I think some assertions would be
> really great, I'm just not sure where.

Yes, on x86=E2=80=99s version of the function.