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

[Hugh Dickins <hughd@google.com>]

On Thu, 2 Aug 2018, Dave Hansen wrote:
> 
> From: Dave Hansen <dave.hansen@linux.intel.com>
> 
> The x86 code has several places where it frees parts of kernel image:
> 
>  1. Unused SMP alternative
>  2. __init code
>  3. The hole between text and rodata
>  4. The hole between rodata and data
> 
> We call free_init_pages() to do this.  Strangely, we convert the
> symbol addresses to kernel direct map addresses in some cases
> (#3, #4) but not others (#1, #2).
> 
> The virt_to_page() and the other code in free_reserved_area() now
> works fine for for symbol addresses on x86, so don't bother
> converting the addresses to direct map addresses before freeing
> them.
> 

Thanks Dave, this series works for me.

But in trying to review it, I am feeling very stupid about this 3/7
(and/or the 2/7 before it: though I don't think that it's wrong).

I simply do not understand how this change works, and how #1 and #2
work. I thought that virt_to_page() only works on virtual addresses
in the direct map: the __va(__pa_symbol()) you remove below was a
good conversion from high kernel mapping to direct map. Without it,
how does virt_to_page() then find the right page?

Is it that there's a guaranteed common alignment between the direct
map and the high kernel mapping, and the ffffffff8....... addresses
pass through the arithmetic in such a way that virt_to_page() comes
up with the right answer; and that is well-known and relied upon by
x86 developers? Or are you now adding a dependency on a coincidence?
Or does it not work at all, and the pages are actually not freed?

Hugh

p.s. if there were to be a respin (I hope not), I notice that in
both 1/7 and 2/7 commit comments, you've said " no " for " not ";
which always makes us pause and wonder if you meant " now ".

> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Kees Cook <keescook@google.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Juergen Gross <jgross@suse.com>
> Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Andi Kleen <ak@linux.intel.com>
> ---
> 
>  b/arch/x86/mm/init_64.c |    8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
> 
> diff -puN arch/x86/mm/init_64.c~x86-init-do-not-convert-symbol-addresses arch/x86/mm/init_64.c
> --- a/arch/x86/mm/init_64.c~x86-init-do-not-convert-symbol-addresses	2018-08-02 14:14:48.380483277 -0700
> +++ b/arch/x86/mm/init_64.c	2018-08-02 14:14:48.383483277 -0700
> @@ -1283,12 +1283,8 @@ void mark_rodata_ro(void)
>  	set_memory_ro(start, (end-start) >> PAGE_SHIFT);
>  #endif
>  
> -	free_init_pages("unused kernel",
> -			(unsigned long) __va(__pa_symbol(text_end)),
> -			(unsigned long) __va(__pa_symbol(rodata_start)));
> -	free_init_pages("unused kernel",
> -			(unsigned long) __va(__pa_symbol(rodata_end)),
> -			(unsigned long) __va(__pa_symbol(_sdata)));
> +	free_init_pages("unused kernel", text_end, rodata_start);
> +	free_init_pages("unused kernel", rodata_end, _sdata);
>  
>  	debug_checkwx();
>  
> _