Re: [PATCH 3/7] x86/mm/init: pass unconverted symbol addresses to free_init_pages()

Linus Torvalds
2018-08-04T17:31:44Z

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