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Date:   Sat, 4 Aug 2018 11:23:34 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
cc:     Hugh Dickins <hughd@google.com>,
        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?Q?J=C3=BCrgen_Gro=C3=9F?= <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>
Subject: Re: [PATCH 3/7] x86/mm/init: pass unconverted symbol addresses to
 free_init_pages()
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On Sat, 4 Aug 2018, Linus Torvalds wrote:
> 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.

Thanks a lot for writing that up. You shamed me into grepping
a little harder than I did yesterday, when all I could find were
"- PAGE_OFFSET" conversions (maybe I got lost in 32-bit-land).
I had missed __phys_addr_nodebug(), where the __START_KERNEL_map
alternative is handled.

Hugh