linux-kernel - Re: [PATCH v2 17/39] mm: Fixup places that call pte

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Date:   Mon, 3 Oct 2022 18:56:15 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc:     X86 ML <x86@...nel.org>, "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-doc@...r.kernel.org,
        Linux MM <linux-mm@...ck.org>,
        linux-arch <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>,
        Borislav Petkov <bp@...en8.de>,
        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>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V . Shankar" <ravi.v.shankar@...el.com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        joao.moreira@...el.com, John Allen <john.allen@....com>,
        kcc@...gle.com, eranian@...gle.com,
        Mike Rapoport <rppt@...nel.org>, jamorris@...ux.microsoft.com,
        dethoma@...rosoft.com, Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v2 17/39] mm: Fixup places that call pte_mkwrite()
 directly

Hopefully I will not waste your time again… If it has been discussed in the
last 26 iterations, just tell me and ignore.

On Sep 29, 2022, at 3:29 PM, Rick Edgecombe <rick.p.edgecombe@...el.com> wrote:

> --- a/mm/migrate_device.c
> +++ b/mm/migrate_device.c
> @@ -606,8 +606,7 @@ static void migrate_vma_insert_page(struct migrate_vma *migrate,
> 			goto abort;
> 		}
> 		entry = mk_pte(page, vma->vm_page_prot);
> -		if (vma->vm_flags & VM_WRITE)
> -			entry = pte_mkwrite(pte_mkdirty(entry));
> +		entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> 	}

This is not exactly the same logic. You might dirty read-only pages since
you call pte_mkdirty() unconditionally. It has been known not to be very
robust (e.g., dirty-COW and friends). Perhaps it is not dangerous following
some recent enhancements, but why do you want to take the risk?

Instead, although it might seem redundant, the compiler will hopefully would
make it efficient:

		if (vma->vm_flags & VM_WRITE) {
			entry = pte_mkdirty(entry);
			entry = maybe_mkwrite(entry, vma);
		}