linux-kernel - Re: [PATCH v5 11/39] x86/mm: Update pte_modify for _PAGE

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Message-ID: <Y+T+ZxydCZS1Yjmz@zn.tnic>
Date:   Thu, 9 Feb 2023 15:08:39 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc:     x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H . J . Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        John Allen <john.allen@....com>, kcc@...gle.com,
        eranian@...gle.com, rppt@...nel.org, jamorris@...ux.microsoft.com,
        dethoma@...rosoft.com, akpm@...ux-foundation.org,
        Andrew.Cooper3@...rix.com, christina.schimpe@...el.com,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v5 11/39] x86/mm: Update pte_modify for _PAGE_COW

On Thu, Jan 19, 2023 at 01:22:49PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@...el.com>
> 
> The Write=0,Dirty=1 PTE has been used to indicate copy-on-write pages.
> However, newer x86 processors also regard a Write=0,Dirty=1 PTE as a
> shadow stack page. In order to separate the two, the software-defined
> _PAGE_DIRTY is changed to _PAGE_COW for the copy-on-write case, and
> pte_*() are updated to do this.

"In order to separate the two, change the software-defined ..."

>From section "2) Describe your changes" in
Documentation/process/submitting-patches.rst:

"Describe your changes in imperative mood, e.g. "make xyzzy do frotz"
instead of "[This patch] makes xyzzy do frotz" or "[I] changed xyzzy
to do frotz", as if you are giving orders to the codebase to change
its behaviour."

> +static inline pte_t __pte_mkdirty(pte_t pte, bool soft)
> +{
> +	pteval_t dirty = _PAGE_DIRTY;
> +
> +	if (soft)
> +		dirty |= _PAGE_SOFT_DIRTY;
> +
> +	return pte_set_flags(pte, dirty);
> +}

Dunno, do you even need that __pte_mkdirty() helper?

AFAIU, pte_mkdirty() will always set _PAGE_SOFT_DIRTY too so whatever
the __pte_mkdirty() thing needs to do, you can simply do it by foot in
the two callsites.

And this way you won't have the confusion: should I use pte_mkdirty() or
__pte_mkdirty()?

Ditto for the pmd variants.

Otherwise, this is starting to make more sense now.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette