linux-kernel - Re: [PATCH 3/7] x86/mm/init: pass unconverted symbol addresses to free_init

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Date:   Sat, 4 Aug 2018 10:31:29 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Jürgen Groß <jgross@...e.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        Andrew Lutomirski <luto@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 3/7] x86/mm/init: pass unconverted symbol addresses to free_init_pages()

On Fri, Aug 3, 2018 at 5:19 PM Hugh Dickins <hughd@...gle.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