On Thu, 2 Aug 2018, Dave Hansen wrote:
>
> From: Dave Hansen <dave.han...@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.han...@linux.intel.com>
> Cc: Kees Cook <keesc...@google.com>
> Cc: Thomas Gleixner <t...@linutronix.de>
> Cc: Ingo Molnar <mi...@kernel.org>
> Cc: Andrea Arcangeli <aarca...@redhat.com>
> Cc: Juergen Gross <jgr...@suse.com>
> Cc: Josh Poimboeuf <jpoim...@redhat.com>
> Cc: Greg Kroah-Hartman <gre...@linuxfoundation.org>
> Cc: Peter Zijlstra <pet...@infradead.org>
> Cc: Hugh Dickins <hu...@google.com>
> Cc: Linus Torvalds <torva...@linux-foundation.org>
> Cc: Borislav Petkov <b...@alien8.de>
> Cc: Andy Lutomirski <l...@kernel.org>
> Cc: Andi Kleen <a...@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();
>
> _
