Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
References: <20180802225823.4711C55B@viggo.jf.intel.com> <20180802225828.89B2D0E2@viggo.jf.intel.com>
 <alpine.LSU.2.11.1808031650050.4436@eggly.anvils>
In-Reply-To: <alpine.LSU.2.11.1808031650050.4436@eggly.anvils>
From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sat, 4 Aug 2018 10:31:29 -0700
Message-ID: <CA+55aFzd-F65GVBq8a-vTaU4G5UwddDcOdcd3SV9YVdB6qDGdg@mail.gmail.com>
Subject: Re: [PATCH 3/7] x86/mm/init: pass unconverted symbol addresses to free_init_pages()
To:     Hugh Dickins <hughd@google.com>
Cc:     Dave Hansen <dave.hansen@linux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Kees Cook <keescook@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Borislav Petkov <bp@alien8.de>,
        Andrew Lutomirski <luto@kernel.org>,
        Andi Kleen <ak@linux.intel.com>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Fri, Aug 3, 2018 at 5:19 PM Hugh Dickins <hughd@google.com> wrote:
>
> I thought that virt_to_page() only works on virtual addresses
> in the direct map

You're right that virt_to_page() does not work on any _actual_ virtual
mappings (ie no user pages, and no vmalloc() pages etc). It does not
follow page tables at all.

And on 32-bit, it literally ends up doing (see __phys_addr_nodebug()) a simple

    #define __phys_addr_nodebug(x)  ((x) - PAGE_OFFSET)

However, on x86-64, we have *two* cases of direct mappings: we have
the one at __START_KERNEL_map, and we have the one at PAGE_OFFSET.

And virt_to_page() handles both of those direct mappings.

Annoying? Yes. And it has caused bugs in the past. And I entirely
forget why we needed it on x86-64.

[ Goes around and rummages ]

Oh, never mind, looking around reminded me why: we want to map the
kernel text in the top 31 bits, so that we can use the faster
-mcmodel=kernel because all symbols fit in sign-extended 32 bits.

Maybe there was some other reason too, but I think that's it.

                   Linus