Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Content-Type: text/plain;
        charset=us-ascii
Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3696.120.41.1.1\))
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: <35EEB9F0-D99A-4664-9628-27029B52CFD1@gmail.com>
Date:   Mon, 3 Oct 2022 16:20:53 -0700
Cc:     "bsingharora@gmail.com" <bsingharora@gmail.com>,
        "hpa@zytor.com" <hpa@zytor.com>,
        "Syromiatnikov, Eugene" <esyr@redhat.com>,
        "peterz@infradead.org" <peterz@infradead.org>,
        "rdunlap@infradead.org" <rdunlap@infradead.org>,
        "keescook@chromium.org" <keescook@chromium.org>,
        "Yu, Yu-cheng" <yu-cheng.yu@intel.com>,
        "dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>,
        "kirill.shutemov@linux.intel.com" <kirill.shutemov@linux.intel.com>,
        "Eranian, Stephane" <eranian@google.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "fweimer@redhat.com" <fweimer@redhat.com>,
        "jannh@google.com" <jannh@google.com>,
        "dethoma@microsoft.com" <dethoma@microsoft.com>,
        "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
        "kcc@google.com" <kcc@google.com>, "bp@alien8.de" <bp@alien8.de>,
        "oleg@redhat.com" <oleg@redhat.com>,
        "hjl.tools@gmail.com" <hjl.tools@gmail.com>,
        "Yang, Weijiang" <weijiang.yang@intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        "pavel@ucw.cz" <pavel@ucw.cz>, "arnd@arndb.de" <arnd@arndb.de>,
        "Moreira, Joao" <joao.moreira@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "mike.kravetz@oracle.com" <mike.kravetz@oracle.com>,
        "x86@kernel.org" <x86@kernel.org>,
        "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
        "jamorris@linux.microsoft.com" <jamorris@linux.microsoft.com>,
        "john.allen@amd.com" <john.allen@amd.com>,
        "rppt@kernel.org" <rppt@kernel.org>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "Shankar, Ravi V" <ravi.v.shankar@intel.com>,
        "corbet@lwn.net" <corbet@lwn.net>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-api@vger.kernel.org" <linux-api@vger.kernel.org>,
        "gorcunov@gmail.com" <gorcunov@gmail.com>
Content-Transfer-Encoding: quoted-printable
Message-Id: <65CF3B29-BF53-4BA3-89D9-5398CEB7F813@gmail.com>
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>
 <35EEB9F0-D99A-4664-9628-27029B52CFD1@gmail.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>
Precedence: bulk

On Oct 3, 2022, at 4:17 PM, Nadav Amit <nadav.amit@gmail.com> wrote:

> 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?

No. I am not paying attention. Ignore.